I play on an Android tablet: model number RCT6773W22.
This has been happening a while, but I've just found a way to trigger it.
If I long-click on an item to top off the furnace with smeltables, something else will disappear. The disappearing items are not necessarily smeltable (enchanted diamond sword), but can be (stack of oak logs). The bug doesn't require a full inventory to happen, though it's easier to see that way. If it matters, all my tests were in the nether, in easy-survival, long-clicking on netherrack, into a plain furnace with no hoppers, which is using a bucket of lava for fuel.
I seem to be working around the issue by avoiding long-clicks when loading furnaces.