I have had the same happen. I don't play with any CC or mods--totally vanilla. I have twin sim toddlers. Twin A cannot get the "study numbers" milestone, twin B wasn't getting the "max movement skill" milestone (though movement was maxed). I had to use a cheat to lower twin B's movement skill back to 4 and try again for the running milestone, which appeared as soon as the movement skill was maxed again. Nothing has worked yet for the "study numbers" milestone on twin A.
**Update** I cheated twin A's thinking skill down to 4 and tried again. Twin A's "study numbers" milestone unlocked upon reaching max thinking skill the second time. So, it looks like there's a fix but it is ungainly. The cheat I used was stats.set_skill_level statistic_skill_toddler_thinking 4. The website I found the cheat on was https://www.carls-sims-4-guide.com/cheats.php#skills.
Hope this helps others!
