Photos does a lot of extra work on import (merging RAW+JPEG pairs, generating previews, database indexing, optional deletion), so my guess is a concurrency bug where a buffer gets reused or a file handle is closed before the copy finishes.
Rare, nondeterministic corruption fits the profile.
They constantly ask for an example project, even if it's something that is easily demonstrated, simply by running existing Apple software, and creating a project, would be a huge pain.
They also ignore reports. Very rarely, I may get a ping on one of my reports, asking me to verify that it was fixed in some release. Otherwise, there's no sign that they ever even read it.
I usually end up closing my bug reports and feature requests, after a few months, because I'm tired of looking at them.
It's clear that they consider every bug report to be a burden. That's a very strange stance, but then, they are not a typical company.
I guess you can't argue with the results, as they have a market value North of 3 trillion dollars, but that does not make it any less annoying.
Support asked me to let them know when a contact vanishing did so they could gather logs from me phone.
Once I was finally able to see it happen, I reached out. Reported that it had just happened overnight. The customer support said it was too long of a time frame for engineers to investigate because "it generates a lot of logs and that's too much to go through". I could not believe their answer.
I just moved my contacts to Gmail and that was the end of it.