Things breaking.

I've seen this issue a few times, although I can't remember it happening with high chairs. I think it has something to do with an interrupted action with the child being put in the crib, or maybe being taken out, where the crib isn't released from use after the sim leaves it. I do remember once that I could direct a toddler in a crib to "go here" and he crawled to a spot on the floor, just kind of dropping from the crib like he'd been at floor level all along, and the crib couldn't be used again. This was before I'd installed Generations, so no imaginary friends, although the toddler did for sure have a teddy bear.

I use MC resets for this too. If there's a sim in another crib that I don't want to disturb, I put the affected crib in the household inventory and then filter to "inventory" when running the reset. That way, only the crib in the inventory gets reset, not the others on the lot.