Infinite Furnace with coal block when I leave it burning while I go to the nether in hardcore

Left a stack of sand burning after using a coal block, went to the nether for 10+ minutes, came back and it was still on maximum. Closed out and restarted minecraft, still burning at maximum time remaining, put a stack of cobble in it and it kept burning that. On hardcore, didn't seem to be the same on creative, but I didn't test it in other game modes.