NBT numbertag being reset

Status
Works as designed
Issue description

Hello! I'm working on a magic mod and the alchemy section is giving me some trouble.

I've made a special brewing stand which is supposed to allow players to stack potion effects into a single bottle. E.g. Combining 2 speed potions will create a potion with speed 2. But it isn't working.

It seems like the crafted potion isn't keeping its NBT number tag. When I drink the potion it gives an effect of "speed 0 with duration 0".

Here are the screenshots of the crafting procedure and the drinking procedure. Would anyone be able to help?

 

Issue comments

"Get copy of item from slot X" gives you a copy.

You add NBT to the copy of your item, not original as this is how itemstack inventory is designed for code reasons.

You need to set new item in slot, where this new item is item copy with NBT set. You will need to use itemstack local variable to save current copy of item, modify it and then set in the slot.

TLDR: You are setting NBT tags on copy of your item