In the latest Unity (19.0.3) I imported the XR plugin and created the default setup according to the documenation. I have enabled Windows Mixed Reality plugin in XR Settings.
My scene now renders like this on my HP Reverb.
My teammate has informed me that on his oculus, using the oculus plugin, it all works fine.
The objects are using the Standard Shader, with only a texture added to the albedo.
Deleting the Unity XR Plugin Management thing and going back to the Enable VR Support checkbox in Player Settings works. Now it launches SteamVR when I click play and works properly.
Seems like HP Reverb + Windows MR + XRPluginManagement doesnt render properly.
I’m confused, Are you trying to use WMR or Steam VR? If Steam VR, why are you trying to enable WMR directly? You might be better served trying to use the new SteamVR XR Plugin.
Did you try this with a simple scene and one simple object to see if it repros there?
Do you know where or what the triangle may be in the scene? What materials are applied?
Do you have a repro project that we can play with?