I’m not sure about the purge thing - just speculation on my part. I would think that there would likely be systems in place to prevent reuse of old usernames regardless.
I’ve seen people suggesting these last active dates were visual glitches but that is a lot of visual glitches over the course of roughly a month and a half - different days and times of days, roughly clustered on certain days. Not all old accounts are affected either that I can see.
It’s a curious thing. I’m trying not to imagine Mr. Krabs digging up graves for treasure (Robux) so to speak but that did cross my mind as a possible reason as well.
Thanks for reporting this issue. We have identified and patched the root cause. We are in the process of fixing the last online times of affected users.