Yikes that is a bug.
I got the issue too, but with my celebrity still with his 2 quirk slots. The other 2 slots (quirkslot3 and quirkslot4) are still unlocked
The issue goes like this, I think can be reproduced easily
1. I got the no-touching quirk, which appears in quirkslot1.
2. Then I got the stan quirk in quirkslot2.
3. Then I drink Quirk-B-Gone potion, to remove no-touching quirk
4. Now my celebrity has quirkslot1 empty and glowing red, stan quirk is still in quirkslot2
5. Add additional quirk with 'testingcheats on' cheat, right click the sim, then add new quirk, in my case: I add paparazzi darling
6. instead of appearing quirkslot1, for some reason the quirk appears in quirkslot3, which supposedly still unlocked!
Serious bug here, Sims team needs to check this!
