I am unable to replicate this in Testing due to material costs and roadblocks to reaching this point, but I just had this unusual situation in the live server.
-1708, 94, -221 Berlyn Township
Titanium Furnace (one on the left)
I placed a smart stack of 500 ores of all varieties from my latest mining expedition in the furnace. I then placed the last of my compacted coal reserves (compact soft, compact medium, compact hard in a smart stack) in the fuel slot to insure I had more cycles of fuel than ore, and logged out for the night. Upon return this morning, I found that only my copper had smelted, and my furnace was locked in an "Unable to Start" state as shown in the picture with plenty of fuel left to burn.
When I removed the coal smart stack from the fuel slot, the furnace immediately began burning again with ~50 cycles left. Replacing he coal causes the furnace to lock up again. It seems that my furnace is bugged out so that it will not allow me to burn more than a single compact coal at a time, requiring me to watch it's progress manually rather than smelt while offline. This is problematic!
I will leave the furnace in it's bugged state should a dev decide to look at it to try and figure out what's going on. Let me know when you're finished with it so I can get back to making Titanium tools!