Barrels delete items when Inventory is full/the item can't enter the inventory rather than throwing it on the ground. 1.12.2 Revelation.

Barrels with pickup upgrade or just barrels?

Regular barrels. With upgrades, without, etc. If the item can't go from the barrel inv to your inv it will delete it, whether it's one item or 10 stacks. Instead of dropping it on the ground like other mods.

Are you sure about that? Works fine here: https://ss.latmod.com/pc/2018-04-17_20.03.48.mp4

Maybe you are in creative mode. In that case, it works as intended, because creative mode picks up items and deletes them

Was on a server in survival mode. I don't really know how to show you but I've tested it across multiple instances with different items, barrel upgrades etc and my outcome is the same.

I'll try and get a video of it later.