What do you mean by 'born during gameplay'?
I often had this issue with sims who were born/aged up to teen as townies. I remember moving Olivia Kim-Lewis into my family as a teen and she only had 2 traits, but it was never an issue for sims I was playing controlling (my teens always got their bonus traits).
To give Olivia her bonus trait, I just went into full CAS mode (the one that lets you change traits). I changed her aspiration to something else, then back to whatever it had generated as, and when I exited, she had the bonus trait. It's kinda cheating, but in my mind, cheating to fix game glitches is mostly fair go.