Today I had some time to play so I turned off mods and tested with a brand new save. The first sim in the household to age up was a toddler who aged up as soon as he woke in the morning, 5 AM.
Yes, it was marked on the calendar for that day along with another birthday (I didn't keep playing once I saw the first sim was going to age up, the other sim with a birthday on that day was still asleep but didn't have an age up action in the queue). When I returned to my normal game (with mods turned back on) the next sim to have a birthday didn't show this problem.
I have a feeling that it may be the first sim to age up after changing lifespan settings. On my normal game I had used MC Command Centre to make a custom lifespan and the first sim with a birthday after that aged up early in the morning on the no mods game I switched to short lifespan and the first sim who had a birthday aged up in the early morning, the next one to age up (even though the birthday was on the same day) didn't seem to be affected.
Yes, the no mods game was started on long setting (as it kept settings from my normal game) I created a family with adults, teen, child, & 2 toddlers. After creating them in CAS I went into build mode to renovate a house to fit 7 sims, then went into settings to change to short lifespan. Both toddlers had their birthday on the same day and were the first to age up after playing for the normal number of sim days for short toddler lifestage. The bug only affected one of the 2 toddlers.
