Okay, I saw this happen last night with one of my girls who aged up from teen to adult.
I've moved in spouses who have only 4 traits instead of 5. And now I've had children born to my household who get a trait or 2 every time it's available, but my girl last night aged up to YA with only 3 traits instead of 4, and I was unable to give her 2 traits, just 1.
So I keep having to waste happiness points to get the MidLife Crisis LTR.
Any other fixes to these?