Resurrection and undeath are fundamentally different. On the other hand, it was formerly explicit that death is the end for the spark. On the third hand, I don't think we'd had an in-lore
resurrection -- as in proper returned from beyond the veil of death good as new. Something something Bolas but honestly I don't know what the true lore for his return in Time Spiral is anymore. We could do it in game since Alpha (
Resurrection), but the lore moved away from the idea, to the point where nonblack reanimation spells were usually flavored of some kind of cheat (
Defy Death) rather than a proper return, or otherwise were left ambiguous.
If the soul is fully restored, I guess that should logically mean the spark as well. Even back when Brady was answering our questions the door was left just a little bit ajar for a lichwalker with the caveat that the phylactery containing their entire soul, separated from the body while they were alive, would technically be the 'planeswalker' while their rotting flesh would be expendable. You could even see Karn as kind of a lite version of this cheat, Urza and Glacian are supposedly in there with him in a spiritual sense thanks to their sparks being stored in the Mightstone and Weakstone, and while they didn't seem to have active cognition, more being facets of Karn, they weren't stored intentionally or completely.
I don't like it. It smells wrong. But I'm having trouble discerning exactly where and why it's wrong.