I've been having this issue for a while now, too. On top of Teens to YA not getting a third trait, they also do not seem to be getting their bonus traits for things like Responsibility and Manners, etc. I'm not sure when it started. Maybe around the time of the L&D pack release or maybe one of the updates between LS and LD?
Things were okay after Lovestruck came out because I've been playing the same file I started with the Lovestruck release, and my teens born during Lovestruck did get their traits okay but now they have children of their own, and this next generation of teens are missing them.
I have all packs and no mods/CC.
