To reproduce this, i placed in a stack of coal and a stack of sand, then i left the chunk went on far away, it most likely unloaded, then i came back, the chunk was loaded and i found the furnace still burning, yet the contents were gone, after i exited the game and logged back in, to it still missing items, after that i destroyed the furnace and items weren't still there, only the furnace. Also to note, the burning icon in furnace display was empty but the furnace block was still playing burning animation.