[Solved] Sigil of Power from weapon does not work in Infernalist's demon form

Upon casting the inherent "Sigil of Power" on some staves, Witch Infernalist's ascendancy passive skill "demon form" cancels, despite it being classified a spell. "Shapeshift" tooltip says "upon casting a skill that is not compatible, you return to human form", but HOW is Sigil not compatible, just because it's a different classes spell? I don't like any of that. I just wanna play my Chaos witch, which is already bad enough. Demon form damage ramps up, but Sigil only lasts a couple seconds, so you only have it in your weakest phase, instead of popping it while scaled up to max stacks.

Shapeshifting removes equipped weapons
Sigil of power is tied to a staff
using sigil of power forces you to equip a staff, and use an ability from said staff
since a staff is a weapon,, and you're currently shapeshifted, the only way to use the skill from the weapon is to cancel demon form to equip the weapon and use the ability
not a bug, that sounds like it's working as intended

Thank you