This question has been almost covered by previous questions, but they weren't asking about this specific interaction.
I understand that the initial power gain from Flames of the Faltine uses the word "additional," so this power gain is stopped by stun, but what about power gained from Master of the Realm of Darkness when multiple allies are damaged by Flames of the Faltine? Are these checked one at a time, and therefore not stopped by stun? Or is all the power gained at once, making it so you only gain 1?