As title says, in latest preview 6000.0.20f1 and later XR is not working correctly in HDRP; more specifically XR renders black screen if headset is not active on start of Unity (Play mode or starting a build). It works fine when headset is active / worn by user before Unity starts.
This is a bad regression, which basically renders XR nearly unusable for development in Unity 6, and i am writing here as no-one seems to look at my reported ticket (IN-85345).