I'm pretty sure Concealment works in Astral for dual-natured beings. Could be mistaken.
Well, the rules seem to be that Pixies are Paranormal Critters, which are fundamentally incompatible with Resonance. They're life forms literally born of Magic. Pixies even Vanish when they die.
There are Emergent Critters, which are basically the Resonance version of Paranormal Critters. They, in turn, are fundamentally incompatible with Magic. So, the reason is 'the rules and the setting say so'. Your group is *free* to change the rules and tweak the setting.
Resonance certainly resembles Magic, but it's more like the opposite than 'a form of magic'. At every turn, the two are totally incompatible (in the rules and setting, which you're free to change
).