Hi everyone. I also experienced this bug over the last week, playing with the HSY pack for the first time (years after release....).
First, I wanted to share a mod I found that helps get around this. I never play with mods but became desperate. It worked, my Sim aged up correctly - more details below. Just remember to enable mods and script mods in your Game Options menu if you, like me, never use mods and aren't familiar: https://www.curseforge.com/sims4/mods/teen-age-up-bug-fix
With the mod, my Sim aged up at her birthday party and received: all of her Parenthood character value traits (the ones for responsibility, emotional control, etc), was able to select a third trait, and got the 'graduated with honors' trait (thought she'd be valedictorian, sigh). **The only thing that did not work for me with the mod is that for some reason the graduation ceremony never showed up on calendar for my Sim, but I think that flaw was unique to my game.** Basically, before my Sim aged up I went to play another pre-made household in Evergreen Harbor where I'd noticed one of my Sim's Teen friends had already aged up to YA before her, and saw a graduation ceremony on that household's calendar... which happened to be scheduled for the same day as my Sim's birthday. At that point I didn't care about the graduation ceremony anymore, but again would've been nice to play as this was my first time using HSY.
Without the mod, I played through her cursed groundhog day of a birthday like 6 different times, and my Sim couldn't get her third trait or any of the Parenthood character value traits or any graduation-related traits, ever. It didn't matter if I used a cake or let her age up "naturally" without the cake. I tried reverting to a backup save about 5 days before her birthday and aged her up early with a cake then, same deal. Never aged up correctly.
Anyway, I saved a few versions of my game where the age-up bug happens, and have kept the modded file separate. Sent a vanilla save to EA_Cade.
Edited to add: Totally forgot to mention when I first wrote all this out -- sorry to all the console players who have precious few options for getting around this and can't use mods. It sucks!! Also wanted to acknowledge having a Sim graduate early as a workaround is especially annoying because most people encounter this bug on, or very close to, their Sim's birthday at the end of the Teen stage. Clearly they're not trying to graduate "early" at that point...