I'm not entirely sure but I have the suspicion that this might be a result of the game's relationship culling.
I once (long time ago) looked up the relationships after an NPC called me and I'm pretty sure I saw a tiny romance bar. But the game is very rigorous with relationship culling especially for NPCs (that's also a big problem in GT clubs imo) so it's quite possible that relationships like these might get deleted.
The only time I actually was able to trigger a changed relationship status was when I used the "hook up" interaction on a Sim in my own household (it was mainly for story purposes). These Sims actually starting dating and where boyfriend and girlfriend afterwards.
It's really sad. I wish this interaction was more meaningful (and that they'd only call for Sims that my Sim actually knows and can have an opinion about).
I don’t think it’s an issue with culling because I experience the same thing and within the Sim world, the time span where it happens is so short, within a few days, that nothing should be culled off at that point. I’m on console with this issue.
