We haven't seen this, but we have run into an old ghost of a problem where some of our photo attachments appeared to be randomly deleted from the database for an entire day—a few days after the day in question.
Last time I ran into this, it was because the organization at the time had a few field workers that had been sharing the same ID, so we just wrote it off as being a result of that and I made sure everyone properly had their own, individual account this time 'round.
So now I'm baffled that the issue is back, on a different project, in an entirely different organization.
The two issues might be utterly unrelated, or there might be some underlying glitch in the attachment database system. If you find anything out about your issue, I'd love to hear about it, in case it sheds any light on ours.