I am at work right now, so I cannot try and reproduce it.
I played Grimm's Grotto this morning with a Half-Dragon (a few classes are missing before it is rainbowed).
I saw Whurrgarbl in the shops, so I bought it (I did not prep Avatar's codex though) since I remembered the
discussion with srid, and I wanted to try it.
I was carefully monitoring the burning stacks, since I wanted to see how it worked.
There was a monster with MAX burning stacks.
I attacked it in melee. Burning stacks reset to 1.
by the way, I had no mace, but i am not sure if I had Binlor's knockback or not. I do not remember if I knocked the monster into a block, empty space, or another monster.