The bug

Prior to 15w49a (see MC-80597), the lit furnace block had a dedicated item form, which was separate from the normal/intended lit furnace item. If a world containing this item is loaded in the latest version (which is currently 1.18.2), the item is deleted from the inventory completely.

While this may seem like a won't fix or feature request since this item was never intended to be obtainable normally, it ultimately was accidentally obtainable in survival in several versions, specifically in a way that could result in them being collected on "accident", much like in MC-249703. Notably, before Alpha v1.0.1, furnaces broken when lit would drop the lit furnace item rather than the regular furnace item. Due to this, there is a very real chance that an old survival world containing this item (which was impossible to distinguish from normal furnaces at the time without placing them) could be upgraded to the latest version, causing several of a player's collected furnaces to mysteriously disappear for seemingly no reason.

How to reproduce

1. Play in a version before 15w49a
2. Obtain a lit furnace via your preferred method (/give command, mining one in Infdev and progressively updating the world, silk touch in b1.9-pre4 and upgrading the world, or something else)
3. If the world is pre-1.2, load it in a version between 12w07a and 15w47c
4. Load this world in 1.18.2 or whatever the latest version is

Expected results

The lit furnace item would still be in the player's inventory. Either it would be converted directly to a normal unlit furnace (as to keep things "legitimate"), or it would be given a BlockStateTag that makes it be lit when placed (to preserve prior behaviour).

Actual results

The item just disappears completely. Not exactly good news if you ended up breaking furnaces when they were lit, stored them somewhere for future use, and came back to them in the current version.