Per the standard.site verification documentation, publications that aren't homed at the apex domain (such as on a many-tenant blogging platform) will publish their AT-URI artifacts to a subpath in the .well-known folder. These are currently not handled.
https://standard.site/docs/verification/#non-root-publications
Fixed in https://tangled.org/paeth.xyz/siteseer/commit/afe2bd30c91f20a04e01639b284a5cd5729711d5