Here is one possible scenario:
The editor invited n reviewers at first. After receiving the reviews of that group, the status became "Required Reviews Completed". Then, for some reason, like lack of consensus between their opinions or even contradictions between their assessments, the editor has decided to invite another set of m reviewers. In the meanwhile, the editor might (inadvertently) set the counter of the required review submissions (before the completion of the review process) to 1 while they have already invited m. So, every time a review is submitted, the status becomes "Required Reviews Completed" because of that counter (which was set to 1), and it again jumps to "Under Review" because some of those m reviews are yet to be submitted.