MC1.12.2
forge-14-23-1-2582
TConstruct-1.12-2.8.1.49
TinkerComplement-1.12.1-0.2.2b

Explaination: Create a melter with a heater. Put solid fuel in it, and use some fuel somehow.
Replace the heater with a lava tank. The solid fuel is still in the buffer, and heat doesn't rise until that buffer is gone. (cannot melt above 200C)

Solution: Removing Heater clears fuel buffer?

Its kinda intentional the way it works right now, though I can see the downsides. I originally figured if you just put big fuel in there (block of coal), you would not want it all wasted, but I guess the higher melting temperature is probably an advantage. For now you could just break and replace the melter and it will clear the buffer

I can definitely see it staying the way now, but I stood dumbfounded for quite a while (about 10 mins) before I fully understood why it was happening.

I think it'll cause less confusion if the buffer clears.

Should be addressed by ce7d4d6, which will be in the next release. I have a few other things which I might try to finish before then though

Added in 0.2.3