I've been doing testing with the movement skill cause I also can't unlock two of my movement milestones. The motor milestones are level based and seems to only trigger one when you gain a skill point. So level 1 is learned to walk, level 2 is learned to climb stairs, so on and so forth. The other ones, like social and cognitive, aren't level based, they're action based (study shapes, study numbers, first friend).
But with the way the movement ones are triggered, if you started with level 1 skill point, you'd have no milestones. Then when you level up to 2, you'd unlock learned to walk and perpetually be behind one milestone.
Edited: It's definitely a bug and it shouldn't be happening. If you want to get the milestone, though, you can cheat the skill down then back up with the
stats.set_skill_level statistic_skill_toddler_movement # cheat.
This seems to fit my experience as well. My sim started toddlerhood with bonus skill exp in Movement from accomplishing stuff as an infant. She has been one milestone behind every time she leveled up. I suspect this only happens with toddlers who have aged up from infancy, and doesn't affect toddlers created in CAS.
