@MyraMierI went back to one of my previous engaged Sims today to check this out. Their friendship bar was high but the romance bar was about half way and they had just recently got engaged with one of those "I found a ring.." phonecalls. So they were okay but I boosted their relationship just incase.
I switched households to my new Sim, Keeli and played her story for a few Sim days and she had friendly relationships drop down a level (which it seems to do more quickly) if she didn't stay in touch within about one Sim week. So I got notifications saying she'd become "friends" with someone she was previously good friends with. She's been in the world in a house share and has a sister but I hadn't played her until today.
I'd maybe test this on another household when you get a chance. I'm going to test it as well on Keeli when I find her a suitable partner. She just started her acting career so it's time for her to start a showmance and see if she has a breakup without trying to.
It might help if you can manage to add an image of the relationship bars here so we can see where it was at. I'll ask a PC player too, since I'm on Xbox Series X so I don't use MCCC.
Edit: if you test it on a different relationship, I'd try one that didn't have the best friends pop up and just starts the regular way and builds with social interactions face to face.
