In UE5.1 I’m having the same problem with my project. Even in the material or mesh editor preview window, assets appear with no diffuse lighting at all, only reflections. I suspect it may have something to do with raytracing features being enabled.
Do you have these project settings enabled?
If so, try disabling “Visible in Ray Tracing” on the HDRI Backdrop Geometry component in the details panel under the ‘Rendering > Advanced’ tab. Worked for me.
