The progress arrow in a furnace, smelter, or smoker stops updating at random points. This does not affect the actual smelting process (1 coal smelts 8 items, for example.)

This happens whether or not player leaves the area.

Two example screenshots attached.

Progress stops during the actual smelting process, but progress indicator does not clear when items are removed. Putting more ore/food AND fuel in resets the indicator, though.