I've noticed this problem too. Seems to happen when the sim doesn't live alone, like, maybe the game mistakes any sort of roommate for a romantic partner ☹️
Actually, it happens even when they live alone. I strongly suspect that the said relationship is literally the one they just got into with the sim you hooked them up with. Because it literally happened to me every single time I tried to hook up two sims. My sim gets a call from the first one announcing they got into a relationship with the second sim. Then the second sims calls that the first sim is already in a relationship. So I assume the game tags sim A as in a relationship with the first call and then it tries to tag sim B, but sim A is tagged as taken and the game somehow doesn't realize they are with sim B? Honestly, I wouldn't be surprised. I usually cheat the angry sentiments and embarrassed moodlets away and then jump over to one of the other sims and hook them up through normal interactions.
The Sims TeamI had this happen recently, tried to hook up Salim Benali and the creature keeper and as I have not played through the cottage living stories I assumed he might be marked as "in a relationship" because something was meant to happen. But I'm pretty sure I tried it on other occasions and it still did not work, I even temporarily added the Sims to my household to inspect their relationships and there was nothing (they were neither in a realtionship with each other nor with anybody else). A save file in which it happened at any point works?
