The title is pretty much exactly the issue. Whenever there is an item with a large amount of enchantments and it you attempt to socket it in a furnace, the item will be have everything removed and be returned to the base item.

I do not know the exact amount of enchants that will cause this, but I am using a tier that has a max of 18, so I'd say 18 would be the absolute maximum. I've also only tested this with 3 sockets as well, as that's the maximum. So I'd say it's impossible to smelt an 18 enchant item with 3 sockets in such a way as you use all of the sockets on the item.

Thanks for your time. 👍