When overstocking a furnace with sponges (transfering over a stack when there are already items in the smelting slot) the surplus items over write the first item in your inventory that has a stack limit of one like a diamond pick or diamond sword. It did not overwrite a stack of 6 bread even though it was the first item in the inventory
Linked issues
Comments 2
Resolving as a duplicate of MCPE-18617.
This is most likely a duplicate of MCPE-18617. Your description allowed me to finally reproduce it where the others failed 🙂
That said, I could only reproduce this using the pocket UI profile; using the classic profile on the Win10 version doesn't show this behavior.