[DU] Graduation ceremony ar different times for the same program and university

I assume different time is by design so that you can choose whether to follow or not for each sim separately.
Sims in the exact same program often have their class schedule differ, so nothing says they must graduate at the same time.

The gllitch you described exists tho, I had up to 8 sims graduate the same day and attend ceremonies set minutes apart with 1 or 2 of them glitching out of it randomly and not getting a portrait.
I doubt same time setting is what would prevent it from happening.