If you leave the smelter running when you go down then sometimes, if your smelting finishes before you come back, you can end up with phantom titanium ingots in the smelter that have to be "smelted" into nothing before you can choose something else. I'm not sure I can give any other information about how this might have happened and it might be a save file thing? (I have only played on this version of the game... Minor Update 29).
Hope this helps <3 - love the game! :) - trying for a hard mode the first time around but finding it tough, I'm probably missing something...!