It's a pair of known bugs, though they occur much more rarely than they used to. One is in the event of a momentary desync (client removes items from character inventory but server fails to receive message that they've been added to smelter - usually only happens for a fraction of a second so only the one interaction is lost). The other is when something keeps the server from updating that smelter's database entry correctly, like if the file has been locked by an AV scan on the server, or the server thinks there's a corruption, or you're in multiplayer and the zone is currently being processed by another player with a subpar connection/processor/gpu... (in this case, the smelter will "eat" everything you try to put into it until fixed - restarting your client will fix the problem most of the time, severe and persistent versions will require destroying and rebuilding the smelter).

The second bug can also affect chests, making them inaccessible for a while (won't open, no matter how often you try).

Restart game?
Do you mod?
On a server?

Sounds like desync