When a villager sells out of an item, he will not regain it back after going into regen mode.

He will only gain back one item even if he is sold-out of 3 items.

I have one villager that is sold out of 3 items.

I have traded with him numerous times to make him restock, but when he restocks, he only gains one item, and not the one i need.

(this may be linked to the fact that converted world villagers STILL won't pick up willingness items, or enter love mode after the recent patch. Although they said they fixed it, it is still broken and seems they might have caused this glitch instead...)