I'm using an LG Nexus 5X (android 7.0, also did the same on Marshmallow), and over the last few weeks I've noticed that sometimes a load of notifications will come in at once(within seconds), say 3, 5, 10. Some quite old (hours). eg 5x facebook, 3x email at once. A couple of weeks ago I had 20+ twitter notifications pour in in a couple of seconds.
This doesn't happen all the time, sometimes after I move or wake the phone, so makes me think its suspending notifications based on motion, if that's a thing?!
It also doesn't appear to be any different on wifi vs 4G, I have no issues with browsing or calling.