It works properly using the OpenXR plugin from what I’ve seen. Unfortunately the SteamVR OpenXR implementation is still far from feature parity with their native API, and Valve seems to have very few people working on it.
I haven’t tested with 4.27 or UE5 yet though.
Disabling running start isn’t really a proper solution even if it was possible to set, since it will limit the frame budget to the render thread and GPU time added together.