It seems to delete non stackable items, just as mine carts, horse armour, etc when shift clicking them into a chest

Another users explanation:
"you put all your not normally stackable item in a chest with shift click, and it tries to stack them, but it gets merged"

Thanks for making this a ticket. As mentioned in discord: This appears to be an api breakage that is causing setCurrentItem to not keep the stack sizes. I am digging into why it would break that way now, but have a potential fix that I'm experimenting with as well.

Hello, great project. I just want to post a few observations that I made tonight:

1. StackableItems works on a 1.21 server running viaversion if you use a 1.20.4 client
2. Changing the stackable items count for brewing stands seems to delete items (might be bc I shift clicked like OP)
3. Not doing so results in potions being deleted when brewing
4. My 1.21 client would also delete items when running on a viaversion 1.20.4 server

I would like to add my observations.
Version 1.3.0
Server: paper 1.21.1 # 13
Stackable items do not stack in chests even if set to in the items.yml. Stacked items merge when placed into chests.
Basically StackableItems has no effect on chests.