Factorio Forums

If a fuel item is an ingredient in a furnace recipe, inserters will not automatically insert it as an ingredient, nor will entity.insert(), ctrl-clicking, or shift-clicking. Players may still manually place the item by dropping a stack in the inventory slot.

Hello, it is a minor thing, and will not be solved until we optimise the insertion logic, so the furnace doesn't have to check recipes every time some inserter wants to insert fuel into it.