I am creating an addon which adds "Steel" as a material in game. I made the nuggets and ingots fine, and even made armor to go with it. I then created recipes to smelt the steel armor down into nuggets, but instead of just 1 nugget I wanted 4 to come out per item so I did the following recipe:

{
"format_version": "1.12",
"minecraft:recipe_furnace": {
"description":

{ "identifier": "mist:furnace_steel_chestplate" }

,
"tags": ["furnace", "blast_furnace"],
"input": "mist:steel_chestplate",
"output":

{ "item" : "mist:steel_nugget", "count" : 4 }

}
}

Now if you smelt a single item, it works perfectly and 4 nuggets are given out. If the 4 nuggets are removed, the next item will also give 4 nuggets. However, if at any time there is any of the nuggets in the output of the furnace when the next item stops smelting, it only adds 1 nugget to the output.

I've attached the pack in case something is wrong but I'm 100% sure this is a bug with the game itself due to the fact it works perfectly if the output is empty.

Replicate Instructions:
Install the pack.
Create a new world with the pack enabled. (Turn on additional modding and holiday creator)
Place a furnace and add fuel.
Smelt 1 steel armor item, then smelt another without removing the nuggets that come out.

Result: total of 5 nuggets given.
Expected: 8 nuggets should be given.