I’m having this same problem. Campfires, Forges, and fireplaces do that 1 stack thing until I take the charcoal out(which it stacks fine just annoying it doesn’t work while in their inventories.). However, the strange part is that charcoal stacks 100% properly in Industrial Forges. It makes no sense. Really trying to fix it. Replacing campfire and other units seems too drastic. Maybe there is a burning item inventory that needs replaced? I’m still look, but there are also two other issues I’m currently looking in to as well.—
Edit: Fixed it. Follow Wookie’s tutorial on campfire. https://www.youtube.com/watch?v=1N04-nyO6ak&list=PL1kfMnmDe3Y6fGPeiIocKbU4PCTs53fjO&index=6
You need to make childern of EngramEntry,Primalitemstructure, and the PrimalInventoryBP_campfire(or forge). For the actual structure which is example: Campfire (blueprint) – you need to make a copy and not a child. Remap the engrams and replace the InventoryBP in the structure component. I had to do this 3 times. Campfire, fireplace, and Refining forge. Also dont forget the most important part of changing your modded charcoal to be used in the structure data.