You indirectly want to access camera buffer.
You have a USceneCapture2D component to capture what user see in your VR scene. If it was just a VR scene, it would be okay but for RGB passthrough, engine have to access front facing raw camera buffer and there is no access for it.
So, you will see your digital objects in front of black screen.
You can see passthrough because Quest device handles it but not engine. So, engine see passthrough as empty area.