I was playing Xander Clavell, who is a grab bag of difficult traits (Hot-headed, Commitment Issues, etc.), but basically against type, as he had been living in somewhat like a committed relationship with an RI for at least a week. I had already planned to finish his Heartbreaker lifetime wish at university, where you have a good mix of "throwaway" relationship targets, but this ended differently. During the last night, he was dreaming of marriage, kids, etc., and the next morning in the bathroom, his lifetime wish simply fulfilled. I don't know the exact circumstances, as I wasn't playing him but his RI who was busy with the garden at that moment. I only heard the typical music. I was somewhat baffled, so I felt like asking, whether this was a bug or a feature?

Probably a bug or a glitch as I had a sim in the Jockey career, and she was nowhere near completing it and it completed.

I just kept track of her races and when she got to the number of simoleans she needed to earn, I considered it completed.

On the other hand, for the Ark Builder lifetime wish, I had all six major pets, 2 of each and all the minor ones, and never could get that one to complete, which was very disappointing as collecting all those minor ones took a very long time and their upkeep took even more.
Although I read recently that the minor pets needed to be in terrariums with the insects left in the sim's inventory, not kept in those cute containers. I haven't got up the energy to see if that works yet.

But as for Xander, maybe he got tired of living the heartbreaker lifestyle and just wanted to settle down with his current conquest?

That's definitely how I'd take that!

Yeah, it was weird. To just take this as a decision to settle down is probably a good way of dealing with this.

I know the text description for the wish says "be the boyfriend/girlfriend of 10 different sims," but is it possible that broken-off romantic interest relationships count too? If so, it could be that one of these relationships declined overnight to the point that the sims were no longer RIs, only acquaintances, and the game just took a bit to catch up. I see that same lag with the Super Popular lifetime wish: either it takes 21 friends for the wish to complete, or else it takes 20 friends and some variable amount of time afterwards.

I don't think that game had enough time passed for any of that to play a role. It was just a little bit over a week in the game, and Xander only had had a single other RI before that (from the first day of that game), which had soured at that point already because of his lovely persona. After he had joined the household, I had done nothing for his LTW. He has never even had a single girlfriend/boyfriend. Still hasn't, by the way.