Ideas
This happened to me as well when I was creating sims. I thought it had happened straight from when I first made them, not after doing cas.fulleditmode, but it's possible I did it with fulleditmode if I forgot to select the option at first. I can confirm it's only for partner and not spouse. When I started playing them, I even had to get their romance bars up and make them partners with the in-game interactions. I was so confused because they wouldn't sleep in the same bed together and I thought I had made them partners, so I even went back with the cheat and tried it again and it still didn't take. I even complained in my group chat when I saw the "become romantic partner" interaction show up...
This was also on my save file with no mods or cc.