Do you have logs? (Are you sure that the wizard who played Firestorm had Hot Flames attached at the beginning of the next round?)
I suspect this is more likely a missing of the distinction of what Hot Flames does. It makes subsequent burning damage penetrating, not the initial fire based attack. (So the initial 4 damage of FS gets no boost from Hot Flames, only the subsequent Burning 2 at the beginning of the next round.)
