@Turjan - Hmm. This still sounds strange.
A teenager with five traits (four base plus one social) has no empty slots. The Uni one doesn't count (unless you have sent them through Uni by way of the Traveler mod and they graduated?) and the fifth base one doesn't count because it doesn't yet exist for them. We can use MC to shove more traits on a sim than that which they have slots for, but that's the reverse of what you are describing.
I'm assuming you do not have a mod from another source in play that increases the number of sims' base trait slots? The only NRaas mod capable of doing so would be Retuner, but it would be virtually impossible to have done this by accident.