As a Sanctum owner, when browsing my Sanctum's stash, I can see a button element link for the Treasure Claim page:
Sanctum Residents can also view this button.
Users are not authorized to visit this link, which I believe is deprecated as of Orn version 3.0.
Actually, it is used, just not nearly as often as it used to be; it currently only applies for random monsters who drop loot tags. It appears on the sanctum page since it's the same as a character treasure page, where it applies. The error here is that it's not currently working, since it's perfectly valid for either a sanctum's owner or residents to claim treasure for a sanctum with a stash. No different than sending treasure to the sanctum separately, really.
Makes sense - thanks for checking it. At least there was some issue still present and I wasn't just completely chasing a ghost.