If you are using a consumable item and swap to another item in the frame you use it, both items are expended even if it is more than one in a stack.

What I expected to happen was...:
I was eating a chorus fruit and swapping between them expecting just one to be used. Another time I was drinking potions and the same thing happened.

What actually happened was...:
When I swapped to the stack of chorus fruit from the single chorus fruit I had in the other hotbar slot not only was the single chorus fruit used up, but the whole stack of chorus fruit in the other slot was also used.

Steps to Reproduce:
1. Have at least 2 consumable items of a similar type in your hotbar (Potions/ water bottles, same type of food)
2. Start using one and swap between them using the hotkeys or mousewheel, it may not work this way all the time however, if not
3. Swap to the second item at the exact frame where you consume the item. If you are swapping from a stack to one item it may just consume one, but if you are swapping from one item to a stack it will consume it all.