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.
I have this same issue after the latest patch, cheating the skill fixes the milestone but breaks the skill as it will never complete via skill progress or cheating.
