Not sure if this is just an inaccurate HUD display, but it looks like Umbral Choir sanctuaries are unaffected by depleted planets. Which almost feels unintended, but hey - cravers are a juicy target for sanctuaries, then!

Before:

After:

I dont have any screenshots, because it was an enemy AI doing it...

But I was going for an elimination victory as the cravers, so I was destroying a lot of planets.

Umbral Choir was able to put new citadels on destroyed planets after I invaded the citadels and then wiped out the planets.

Yes, Umbral Choir are unaffected by Depleted planets. Concerning gameplay and balance this decision was made because they have fewer options to restore them than other factions (e.g. No Sowers on Umbral Systems), lore-wise it can probably be said they are unaffected by it because of their slightly detached relation to physical reality.

The same is true the other way around: Custom Umbral Choir with Extreme Foremen will not deplete planets, so they can't grief other empires.

Regarding Umbral Choir placing sanctuaries on obliterated systems: That is intended. The Umbral Choir have a technology to "Colonize Destroyed."