If I should speculate.. The potatoe was put in the cooking slot from the absolut beginning, when I created the fireplace with the rag. What if the burnt counter started counting time from when it was put in the cooking slot, or something like that. While the temperature counter was functionting properly.
When you start a fire it takes some short time before you can cook. I was just away right after igniting, closing doors and back at the fire in like 4-5 seconds. Doors are right next to the stove.