So, when one defeats Vanaduke, the fire tiles are supposed to disappear. They do disappear, that part does work.
(I believe) The part that doesn't work, is that when defeating Vanaduke while he is smashing, new fire tiles will spawn after the old fire tiles have disappeared.

I have seen this behavior before, but the reason why I'm writing this up is because this time I got killed by the new fire tiles while solo and I had to quit after I beat Vanaduke because I'm not going to revive for 40 energy (tokens and the treasure were not worth 40 ce). I defeated Vanaduke through using a double Plague Needle charge attack enabled by Vanaduke stunning me with his smash attack. It was glorious, but the glow of victory hid Vanaduke's cunning revenge q.q

Please either clear the fire tiles again, or clear the fire tiles a bit later.