Ideas
Never cc or mods, Win 11 PC. I, the sim visiting, is friends with parents of infants and toddlers. I will watch infants get "milestones" they already have. I know because I got both infants all milestones completed before I played a different household. I see no notifications except for what the infant does so I know that's what happening. The infants still (even after an obvious milestone action) will not sit, standup, scoot, crawl, eat in the highchair, dance, nothing... they just lay their waiting to be cared for. I play rotationally, I hear that doing so does make a difference, I have the neighborhood stories off except for friends/romance. The same thing occurs when other family members visit the infants, or if the infant(s) and their family are invited over to another family members or friends home.
I also do not have all the packs. if you want to know which one's I have, please tag me and let me know, i;ll provide a screen shot.