Oh well, I'm not sure. I've never done that. You could always try resetting the sim properly by enabling cheats. But keep in mind that enabling them makes you unable to earn trophies on that save file.
I have this problem with my teen vampires - who were turned while teens and are now turning into young adults. The seasonal calendar says it's their birthday, the game tells me to throw a party, but the party planner says nobody is having a birthday. Can only assume the game can't tell the difference between vampires that are still aging up and the ones that can't. Either way, you can throw, and plan in advanced with seasons, every other type of party, but not a birthday. That said, nothing prevents us from throwing a normal house party and serving birthday cake. At least some of the guests will still gather around the birthday sim when they blow out candles no matter what the occasion is.
EDIT: forgot to say: I'm on PC and the vampires have been bugged to all hell since release.
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