Forum Discussion
Still happening in 1.107.151.1020, new save, no mods.
The root of the issue, as OP points out, is that the Learn Numbers/Teach Numbers (Flashcard) interactions have an upper skill limit and become unavailable after the toddler has reached a certain level.
The Numbers milestone cannot be completed with a skilled toddler (level 5).
But the Letters milestone requires level 5 (that's when the interaction unlocks).
The Letters milestone appears as a 'suggested' milestone first. Following this suggestion would lock the toddler out of the Numbers milestone.
A possible workaround is to lower a toddler's Thinking skill to level 4 with stats.set_skill_level statistic_skill_toddler_thinking 4 to have them complete the milestone. Not a solution but might help move things along in a legacy or super-sim challenge.