I have this issue too. I wanted two sims to match with each other, just because I thought it would be cute if they could ask each other out via the app. Sim A showed up in Sim B's Cupid's Corner, so I saved him. Sim B, on the other hand, doesn't appear for Sim A at all. After having tried for days I downloaded a mod that made it so that I can update matches over and over, with no countdown. So I started to do that, and Sim B never appeared, what happened was that there were just more and more randomly generated sims, until it was only randomly generated sims. I've updated the matches well over 30 times and it only gives me randomly generated sims now. I assume this means that it ran out of existing sims, but that would mean that it wouldn't be possible to have the same sim appear twice. I understand that it would be annoying to get the same sim appear twice, but at the same time, when we run out of sims, why not let them appear a second time?
This being said, Sim B never appeared the first time around anyway. I saw that someone wrote here that they won't appear if they're a celebrity or have ever been a celebrity. I don't know if I, when I created this save file, six or so years ago, forgot to turn off the setting that makes everyone a potential celebrity. If that's the case, I assume Sim B was hit by this setting and technically counted as a celebrity for a little while. Is there anything I can do to reset that? Not even sure if that's the case, just thought I could try to reset it if it's possible somehow.