TryGetBoundaryPoints() in XRInputSubSystem returns zero points

[plmayer]

Setup: An original Vive on SteamVR. Unity 2019.4.21f1 with the "new" XR system in use (XR Plugin Managent package), plus the (manually) installed OpenVR XR Plugin from here https://github.com/ValveSoftware/unity-xr-plugin/releases and enabled it in Project Settings. Project is run, Guardian established and showing in the headset. Then this runs:

   List<XRInputSubsystem> lst = new List<XRInputSubsystem>();
   SubsystemManager.GetInstances<XRInputSubsystem>(lst);
   for (int i = 0; i < lst.Count; i++) {
     List<Vector3> bps= new List<Vector3>();
     bool v = lst[i].TryGetBoundaryPoints(bps);
     Debug.Log("TryGetBoundaryPosts gives back " + v+ " with " + bps.Count " points.");
  }

I get one subsystem ("OpenVR"), and I get a "true" for TryGetBoundaryPosts, and a zero for "bps.Count".

When I go back to the "old" Unity system which only used the SteamVR plugin, I get 4 points back on the same setup using OpenVR.Chaperone.GetPlayAreaRect().

Any help?