itemGetContainerItem function deletes the given item if the custom item is in stack of >1.

[GrandeMalum]

Hello, dear developer. Unfortunately the itemGetContainerItem function is somewhat bugged. This example script works fine at first and loads without any issues, but as soon as the Milk Bowl gets placed in the crafting grid in a stack of more than one, then upon being used in a recipe the returning container item (minecraft:bowl) never appears in the inventory or never drops on the ground if the inventory is full. It just disappears.

#loader contenttweaker
import mods.contenttweaker.Item;
import mods.contenttweaker.VanillaFactory;
val milk_bowl = VanillaFactory.createItemFood("milk_bowl", 0);
milk_bowl.alwaysEdible = true;
milk_bowl.healAmount = 0;
milk_bowl.itemUseAction = "DRINK";
milk_bowl.maxStackSize = 16;
milk_bowl.saturation = 0.0;
milk_bowl.itemGetContainerItem = function(stack) {return <item:minecraft:bowl>;};
milk_bowl.register();

[GrandeMalum]

I guess the best example we have for 1.12.2 version is the Honey Bottle functionality from the FutureMC mod. It does everything related to container returning without any issues. https://github.com/thedarkcolour/Future-MC

[GrandeMalum]

I'd also like to add that currently there is no way to make custom food/drink items give back something in return upon being consumed, respecting the stack size, fullness of inventory and other small details. Keep in mind that it's possible to do, but it would require making a large set of scripts which would be just a workaround nonetheless.