Apologies if this has been asked before, but the closest related thread I found was this one, which is about 8 years old, and I thought it might be a bad idea to revive it.

I have a couple in-game who woke up only to find out that one of them lost all of their relationship ties with the other and disappeared from both of their family trees; I realized that something was wrong when the affected sim woke up because they thought there was a stranger in their bed, even though they'd been married to said stranger for several rotations. They both had intact memories, including their dream dates last rotation, but the sim in question otherwise had no idea who this other person was and when I tried to initiate an interaction, Sim A's panel revealed a 0/0 stranger relationship with Sim B, who had 100/100 best friend/love/spouse relationship as normal, and ugh, the sound of my own broken heart and Sim B's was almost too much to bear. I could practically hear, "I just don't love you no more," playing over the scene.

Thinking back on the previous households of the rotation, Sim A never appeared as a walk-by or visitor (that I recall), but another playable called Sim B on the phone. I'm not sure if that had something to do with it, but maybe something went awry during the daily decay procedure while she was spoken to on the phone. Quitting without saving, I went to SimPE to check the neighborhood files and nothing seemed amiss, except that yes, Sim A somehow obtained relationship amnesia and last rotation's back-up revealed that it happened sometime during the present rotation.

Out of worry that Sim A had their flags messed up and was engaged/married to someone else as per another type of game bug, I scoured through their relationships but everything else seemed fine, so I fixed it up in SimPE, ran HoodChecker (which didn't detect anything unusual), and went back to their house, where Sim A suddenly rolled the want to kiss their spouse as if nothing had happened (or woke up from the nightmare that actually occurred).

Should I be concerned about this? The old thread I linked to mentioned that it might've been related to the general corruption of the default EAxis neighborhoods, but the comments also say this might be just one of those TS2 bugs that happen every now and then. This is a custom hood built without any premades, so perhaps the latter could be the reason. This sort of thing happened on a now-defunct hood on another computer before, so it's not something entirely new to me, though I'm wondering if this is cause to revert to a back-up or some other remedial methods.