The seem to re-pop for me whenever I've got a reset due to a hack update/patch/install.
Yes, mine re-pop back to the second trimester once back in their own houses too. I tried even forcing an error on the visitor, but that didn't do anything.
I hate it when they truly aren't even showing yet and they come to a community lot full-term.
That would suck! I haven't seen that one yet!
Thanks for the comments and sympathies.