Why my amount decryption fails sometimes?

Question

Thanks to great answers to this question I managed to improve my output recognition code, however I still have issues with some payments. The amounts are garbled, which means the xor mask is probably not computed correctly.

I guess I'm missing something very simple or external libraries I'm using have some glitch.

Here's my Python code snippet sprinkled with full debug.

print("      secret view key: {:s}".format(binascii.hexlify(svk).decode()))
print("checking tx: {:s}".format(self.txid))
print("tx has {:3d} outputs".format(len(self.outputs)))
print("tx has {:3d} keys".format(len(self.tx_keys)))
for output in self.outputs:
    print("checking output {:d}".format(oidx))
    keyidx = -1
    for tx_key in self.tx_keys:
        keyidx += 1
        print(
            "  checking key {:d}: {:s}".format(
                keyidx, binascii.hexlify(tx_key).decode()
            )
        )
        hsdata = b"".join(
            [
                ed25519.encodepoint(
                    ed25519.scalarmult(
                        ed25519.decodepoint(tx_key), ed25519.decodeint(svk) * 8
                    )
                ),
                varint.encode(oidx),
            ]
        )
        Hs_ur = sha3.keccak_256(hsdata).digest()

        # sc_reduce32:
        Hsint_ur = ed25519.decodeint(Hs_ur)
        Hmul = Hsint_ur / ed25519.l    # info value only
        Hsint = Hsint_ur % ed25519.l
        Hs = ed25519.encodeint(Hsint)
        #

        k = ed25519.encodepoint(
            ed25519.edwards_add(
                ed25519.scalarmult_B(Hsint), ed25519.decodepoint(psk),
            )
        )
        if k != output.key:
            print("    no match")
            continue
        print("     Hs_ur      = {:s}".format(binascii.hexlify(Hs_ur).decode()))
        print("     Hs         = {:s}".format(binascii.hexlify(Hs).decode()))
        print("     Hint_ur    = {:d}".format(Hsint_ur))
        print("     Hint       = {:d}".format(Hsint))
        print("     Hmul       = {:.2f}".format(Hmul))
        amount_hs = sha3.keccak_256(b"amount" + Hs).digest()
        print(
            "     amount_hs  = {:s}".format(
                binascii.hexlify(amount_hs).decode()
            )
        )
        print(
            "     enc_amount = {:s}".format(
                binascii.hexlify(output.encamount).decode()
            )
        )
        xormask = amount_hs[: len(output.encamount)]
        print(
            "     xormask    = {:s}".format(binascii.hexlify(xormask).decode())
        )
        dec_amount = bytes(a ^ b for a, b in zip(output.encamount, xormask))
        print(
            "     dec_amount = {:s}".format(
                binascii.hexlify(dec_amount).decode()
            )
        )
        int_amount = int.from_bytes(
            dec_amount, byteorder="little", signed=False
        )
        print("     int_amount = {:d}".format(int_amount))
        amount = monero.numbers.from_atomic(int_amount)
        print("     amount     = {:.12f}".format(amount))
        amounts.append((oidx, amount))
        break
    oidx += 1

The failing examples are on stagenet wallet 56eDKfprZtQGfB4y6gVLZx5naKVHw6KEKLDoq2WWtLng9ANuBvsw67wfqyhQECoLmjQN4cKAdvMp2WsC5fnw9seKLcCSfjj.

First, the amount should be 1.1 XMR. Receiving address is 76F1zQpev8aKYiZa4RiykvMgpbpavDVHNCJhYa71zB88MquS8vmjPytLSC2nDoBoHX5WpXLR5SyS6ExGbSMVV1nkT6rfZTd with (major, minor) = (0, 3).

secret view key: e507923516f52389eae889b6edc182ada82bb9354fb405abedbe0772a15aea0a
checking tx: 050679bd5717cd4c3d0ed1db7dac4aa7e8a222ffc7661b249e5a595a3af37d3c
tx has   3 outputs
tx has   4 keys
checking output 0
[...]
checking output 2
  checking key 0: 32f30dc0fe0c0ecb7918ba654fcc73b8d74b2d8653ffbbd284475ef3a3dcbea0
    no match
  checking key 1: 7b2c2aadfbfd0b3cbcca0b09e6d445410caf0ea8beb3b66c5573f1eb86515943
    no match
  checking key 2: b16f348827c34bb9620efc65291ca1fb994ab01db5c103aeb9435e6e50d89854
    no match
  checking key 3: e7b4e99f8c3eb19a7a6bcbf205e9441acb5e0193186fae5add2c3394e0e8fc33
     Hs_ur      = b38365a00459da5926204ae221a39ed0032c9a6214c127eea3a182e525c6a6df
     Hs         = aac0e9e7ad50ebe04329b79bd3f24bc1022c9a6214c127eea3a182e525c6a60f
     Hint_ur    = 101160429414286561058410493824695951988207185202632588040406292122661588140979
     Hint       = 7079356908967152276759068505137026857064672530693789162380929924950682878122
     Hmul       = 13.98
     amount_hs  = 78121283ad4069dc1918842b5935ad12aaec9110e87728961dd63a32e8e7fdcb
     enc_amount = 58f647750234eeda
     xormask    = 78121283ad4069dc
     dec_amount = 20e455f6af748706
     int_amount = 470472985184429088
     amount     = 470472.985184429088

Second, where 1.0 XMR is expected. Address is 797WaaL2Tm4CKQcKjLCdtx9Mg6wVuLQcYe68Ljb17euvEMNQDBqXAXN7eHFNy4uSgwJk2hicLnvWY6p4fbAqcfaVEiox2rd of index (0,1).

secret view key: e507923516f52389eae889b6edc182ada82bb9354fb405abedbe0772a15aea0a
checking tx: 9e1650ef843c028b1b5f880e6e7ec6562441e9f723d4cfc515207c507e2c752f
tx has   7 outputs
tx has   8 keys
checking output 0
[...]
checking output 5
  checking key 0: 7f499f5f06de78e6b7399a91b36050e88292f5f62e5681e7845fd54b415358c5
    no match
  checking key 1: 73b8efdba4f3c0e2c9a372105cca4fc94f62599ab99d1d75e0893d4160a2d824
    no match
  checking key 2: 9f7d69600d357bef31163ef770bebe2f35898ddbcb2cfb3f17433dc65a20acee
    no match
  checking key 3: be503a4ad8186a587cd08c6abb74ea5c58119f6a38e504a423eed563a84e3720
    no match
  checking key 4: dae9c26ff80dbddb2f4fdbcb2dc42e106598988214e5acfeed60084ff71e7ca2
    no match
  checking key 5: ce4692824aec372b248e0aa3b61f08d082d698e80b2b0d71f5b89485c6098287
    no match
  checking key 6: 470cb29c97e354bb987b2e4abd5f321981f1d97feda32ed0101375efb62e1506
     Hs_ur      = 1d49aae57466ecf912a94503731effc61a5b9d1d5293b8f96969087e43341d8b
     Hs         = b5a9fbfda14d593960c288eb7d4f07201a5b9d1d5293b8f96969087e43341d0b
     Hint_ur    = 62923085228351870090926105142326278992011507504216180910171871531129174116637
     Hint       = 5027040609693772379140612637982325065154576629176920062156264024845540108725
     Hmul       = 8.69
     amount_hs  = e89bf8f4d6e281ec2fd24b102526d7d754b5294cdeead73450a16c46827a0350
     enc_amount = 1609fc1cf655d9ad
     xormask    = e89bf8f4d6e281ec
     dec_amount = fe9204e820b75841
     int_amount = 4708714762375303934
     amount     = 4708714.762375303934
checking output 6
[...]

And third. This one has 3 outputs belonging to a single wallet's subaddress. One of them fails, with expected value of 0.2 XMR. The receiving address is 7AAMoYytsGDGwVyCrLo3rU1vKwbDM9qHzEapbqUz596HhBEBj2aF4a9dmqaNaAd9LYcQHXTGaZe8KGBZHKi4k1FQSnJB3fa of index (0,5).

secret view key: e507923516f52389eae889b6edc182ada82bb9354fb405abedbe0772a15aea0a
checking tx: e2871c4203e29433257219bc20fa58c68dc12efed8f05a86d59921969a2b97cc
tx has   4 outputs
tx has   1 keys
checking output 0
  checking key 0: 2105e965fe7529abaa9fc3c3518781589b9a55f8394aea8349c08f259d21a011
     Hs_ur      = b5d32c1b921dbc8ba9eb87858ffa322db856195f4662ab7f881c85d375267bc7
     Hs         = 99e4a6bf5578df6a9d91ece11f44bf32b756195f4662ab7f881c85d375267b07
     Hint_ur    = 90227844499988343076557049979814408693606878134973414102304889890092333454261
     Hint       = 3383777572001196508878811223298477803321481822414522830281478630666882442393
     Hmul       = 12.47
     amount_hs  = d825027477697301c41799884d1c85c21f16424ad77772303f03a0a21cb668df
     enc_amount = d8cd743c60697301
     xormask    = d825027477697301
     dec_amount = 00e8764817000000
     int_amount = 100000000000
     amount     = 0.100000000000
checking output 1
  checking key 0: 2105e965fe7529abaa9fc3c3518781589b9a55f8394aea8349c08f259d21a011
    no match
checking output 2
  checking key 0: 2105e965fe7529abaa9fc3c3518781589b9a55f8394aea8349c08f259d21a011
     Hs_ur      = 07b71d3af42a786d69779c30713d8a28573611bc5b64ca8d125aeb0d28d362dd
     Hs         = fef3a1819d2289f4868009ea228d3719563611bc5b64ca8d125aeb0d28d3620d
     Hint_ur    = 100135747897593070915110917150315415114983223372346125091948185109869432256263
     Hint       = 6054675392273662133459491830756489983840710700407326213922822912158526993406
     Hmul       = 13.84
     amount_hs  = af9a2a62843a4e60a0ce1b9e1d8015e3078891b9cfa990d1bbab3a01e6657129
     enc_amount = af224ebbc13a4e60
     xormask    = af9a2a62843a4e60
     dec_amount = 00b864d945000000
     int_amount = 300000000000
     amount     = 0.300000000000
checking output 3
  checking key 0: 2105e965fe7529abaa9fc3c3518781589b9a55f8394aea8349c08f259d21a011
     Hs_ur      = 81493a64f4b36e2f8ff465fc2fb14d478cfe2b4bc87585cc2d3c462953ba35da
     Hs         = 7886beab9dab7fb6acfdd2b5e100fb378bfe2b4bc87585cc2d3c462953ba350a
     Hint_ur    = 98699129852428100445151195285318558098375685193814448536205178562327051127169
     Hint       = 4618057347108691663499769965759632967233172521875649658179816364616145864312
     Hmul       = 13.64
     amount_hs  = c4bd91f8fea5c96250ae1494039fcd97ba0cadf3b71fce420b3a789d766920b3
     enc_amount = b4547ae104c58adf
     xormask    = c4bd91f8fea5c962
     dec_amount = 70e9eb19fa6043bd
     int_amount = 13637850723900975472
     amount     = 13637850.723900975472