Furnace Fire Meter Inaccuracy

I put 1 coal block in a furnace to smelt a stack of irons.
I walked away some distance so the chunk unloaded.
When I came back, the stack of iron had almost finished smelting but the fire meter was still at full fireness.
So I got a stack of cobble out of a chest, and the fire meter was still full when I re-clicked on the furnace.
I left it for about a minute while doing other things, in the same chunk.
When I came back, the furnace was already out, and only two of the cobbles I'd just placed in it had smelted.

This happened before when I was using lava buckets, probably in 1.2.3 - 1.2.5ish Minecraft version.