All right, so for my case with Don specifically, it turns out the issue was the telescope on his roof. Maybe it's supposed to have a scripted abduction or something, I'm not sure, I had never played Pleasantview before. But the telescope is definitely corrupted somehow, as soon as he uses it he resets and then starts having all kinds of glitches, including the missing memories (and lot loading issues too, so this might be the cause for the other person here who had problems with Don). After the Feb. 4. patch I completely reinstalled the game (resetting Pleasantview wasn't enough) and started with deleting his telescope when first loading his lot. Played over 2 sim weeks with no glitches and all memories. :)
Since abductions are fixed in the Feb. 6. patch, it might be worth trying a clean Pleasantview and using the telescope, but I'm not even sure that's the problem since I haven't heard of a scripted abduction for him even though Vincund's is common knowledge. So maybe the telescope is bad for another reason...? Bella was supposed to have been abducted from there so maybe it's to do with that... In any case, I'm not too keen on an alien plotline for Don and don't want to lose his progress now so I won't be testing any more.