Have you tried removing their degree and dropping their social group level (the ones that came with uni) below 8? I'm guessing if anything the game is seeing 'this sim has a degree so they have a trait slot to fill, and they're over 8 in this social group so there's another slot to fill'.
Thanks for your response. With Nraas I've tried dropping the Graduation and Social traits, prior to saving my own CAS sims I've played (although Mortimer's additional traits are not allocated as Social or Grad traits, nor does he appear to have a degree), to no avail. Next time I save a played and seasoned sim, I will attempt to delete the sims Uni Degree with Nraas/MC/Career - I'm not sure whether there's a similar setting for erasing Social Group history? Mortimer has no academic or social group history, so far as I can tell.
If you are also using NRaas StoryProgression, you could direct it to stop automatically filling in traits that it sees as "missing." The downside is that you might end up with other sims in the world who have less than a full set.
Thanks Igazor - I didn't see your reply before I replied to Orose. That sounds like it should do the trick - I tend to to keep tabs on the traits sims have in my worlds, so I can manually fix any who are trait deficient . You were really helpful - thanks again!