The current volume usage is only available under the specific condition that kubelet endpoints report the necessary volume metrics. In the case where these metrics are unavailable, the service will default to simply reporting the configured capacity level of the persistent volume claim.
In my use case, this seems to work when using a local disk of the node system, but be unavailable when using a hoster-provided volume.
I wonder if someone is aware of a method to monitor PVC utilziation even when the kubelet, which is apparently a node-based control process, endpoint does not report it.