Working on a custom map, I decided to give the player enchanted coal. I wanted to use /testforblock to detect furnaces with that enchanted coal and lower its CookTimeTotal. It worked at first. However, once the furnace consumed one of the enchanted coal items, it broke. Specifically, the furnace was no longer detected by /testforblock, giving the strange error message, "The block at <x,y,z> is Furnace (expected: Furnace)." If it's expecting furnace and it found a furnace, why is it failing?

A bit of inspection shows that after the furnace consumes one of the enchanted fuels, the "ench" tag on the fuel slot gets flattened--in other words, rather than an array of enchantment objects, it becomes a 1D array of values. So if the original tag was "ench:[

]" it gets converted into "ench:[0:32,1:1]" after consuming the first piece. That's strange, but it still doesn't explain the failing /testforblock behavior.

To reproduce:

1) /give yourself a few pieces of enchanted coal using the "ench" tag.
2) Put that stack into the fuel slot of a furnace and start smelting something, allowing it to use at least one of the coal.
3) Try a /testforblock x y z minecraft:furnace command and watch it fail with an odd message.

I realize enchanted fuel isn't something that's natively supported, but surely it shouldn't break all future /testforblock commands until the furnace is broken and replaced, right?