So I tested without mods and CC, the issue still seems to be happening. Hopefully I tested properly!
Huh!
In my normal, modded game, the milestone just unlocked!! No idea why it suddenly unlocked now, when it hadn't previously. The pee appeared and everything. Maybe I'm just impatient? I'll have to go to my other save where I recently had infants (who are both toddlers now) and check if they got the milestone or not as I don't remember.
Previously, my caregiver would react as though she'd been peed on and would get the associated tense moodlet, but there was no actual pee animation and the milestone didn't appear. As for the infant who had the "Free Air Tinkler" quirk that was hidden, I used testingcheats to remove that quirk and add in the "Loves Sounds" one instead.
How weird...
So I checked my other save where I recently had infants. The older sibling did have the Peed on Caregiver milestone, but the younger one didn't. I think the older one aged up to Toddler before the January patch, though.
