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.
I start my first day @ Apple in a few weeks, so I ACK that my opinion might be a little biased here.
(Yes, this came close to killing someone close to me. Fortunately someone else happened to come along to help.)