Mass trading can delete items.

[Kaleopsis]

I like to use this mod when I'm playing on a mod that lets you pick a class to bypass villager trading limits, and whenever you just start shift clicking the trades and start trading a ton of items the majority will be deleted for some reason if you don't slow down and only shift click every couple seconds. It can also delete items when you're trading with custom villagers with weird trading quantities, even if you're only trading a few times at once.