I was in survival, and I had put in a block of coal in a furnace along with a stack of iron ore. Around that time, I had crashed (without a crash report), and I later noticed that it was smelting my ore without the flame dying down. Upon further investigation, I saw that it started down and died when the number of smelts remaining was less than one, which shows it did give exactly 80 smelts worth of time. I could not replicate the issue, but I believe it was related to the crash.

Steps to replicate:
1: Put smeltables and coal in a furnace.
2: Crash the game, hard.
3: Wait to see if the flames die down.