Wow, are u really 1000% sure about this ? Because in game standing still, doing nothing but generating fire aura, preparation is consumed. It is consistent with the description of the node “you have a chance to cast the aura”.
So for me both with the node description plus the in-game tests, preparation node is consumed by those casts.
Then that’s a bug. If you’re not using a spell then Preparation shouldn’t be consumed. 'Cause Fire Aura is not a spell you can cast. Unless it’s being consumed by the melee skill you’re using to generate fire auras while standing still.