Item loss bug Furnace+hopper

Here is an automatic furnace design but it loses a lot of items into nothing.
The fuel (in this case Coal) sometime uses at random 65 items instead of 1.
And sometimes if i put 8 or more in it only 7 will be processed and the rest will vanish.
Happened both in Survival, Creative Singleplayer and Multiplayer and 2 different worlds.