Coal doesn't smelt exactly 8 items.
If you put 8 input and 1 coal, it smelts fine. But if there are more input like a full stack, then the issue occur.
For example you put a stack of sand and more than 8 coals in a furnace, it will cost the 9th coal to smelt a little of the rest. Another is to put a stack of item and 8 coals. You will find there are 63 output, 1 input left and no coal. Really annoying.
iOS 11.3. MCPE 1.2.14 tested OK