What GPU are you using?
In case anyone else having trouble I finally got my AMD GPU to work with HP Reverb G2 both in SteamVR and in OpenXR.
The steps I performed
Apply all windows update
Removed all OpenXR apps / tools from PC
Uninstall all graphics driver from my PC using Guru3D DDU. This include both old Nvidia and AMD drivers
Ran Windows update which detected AMD GPU driver
Install Radeon suite latest version
Open Device manager under windows. Show hidden device. Disable virtual displays.
Update to X-Plane 12.06b4
Ran X-Plane 12 with open XR flag.