Had this happen to 3 sims so far. First time I was taking my sim to LuLu Lounge in Downtown for a date and he errored out while the lot was loading, so he couldn't be reset and was gone when I rebooted the game. Remade that sim and redid what I had done, didn't go back to that lot.
So far so good until I had to move him and his family. After the move his newborn twin daughters were gone even though they showed in the loading screen when entering the lot initially. Checked OneDrive to see if that could be it but turned out I'm not even signed into it on my PC. Antivirus hasn't flagged anything or quarantined anything (it clearly tells me when it has). Twin girls come up in both the mum and the dad's memories as "Had §Subject!"
Because the first sim I lost was the only one I had, it produced an error report, so I've attached that. The daughters didn't produce error files unfortunately. Considering how widespread the issue is I won't bother with a dxdiag file. Hope they get onto this issue quick as this and the infinite loading screen resort bug are the only two major bugs I've encountered so far, but the resort one at least has an easy temp fix.