Ideas
This bug is due to a stuck graduation event. It won't happen unless you somehow get one that is marked as in progress, but never completes. I haven't found how to create the stuck graduation event, but I do have a save where that happened. No mods or CC ever used in that game. From now on, all teenagers aged up with cakes get no 3rd trait or graduation notification and I get the LE mentioned elsewhere. Some of the things I have done:
- Aged up a teenager to young adult and then switched households before graduation.
- Aged a teenager to young adult and then moved the teenager out of the household before graduation.
- Imported a new family into the game with a teenager and then aged them up into a young adult with a cake.
- Imported a sim that was previously a teenager in the gallery into CAS and aged them down to a younger age. Then aged them back up to teenager later with a cake and to young adult.
- Aged up teenagers to a young adult while at a prom event. 😁 Don't ask why.
- Aged an NPC teenager up to a young adult with a cake.
Maybe one of those things caused the glitch. I do have lots of backups for this save file at various points since I was playing a Super Sim challenge and recording it. If you need an example of when this happens let me know.
TEMPORARY work-around: Yes, I know it sucks, but if this does happen to your game, it will always happen from that point on. The only way I have found to fix this is to start a new save. Import all of your old lots and households from the old save and start fresh. That will fix it until it happens again.