Hi there,
I have the same problem.
The issue is that vr.PixelDensity is not implemented in the OpenXR plugin yet.
This affects the OpenXR implementation prior to Unreal 5.1 which is not released yet.
You can see though that there are 2 commits done in the current 5.1 branch:
https://github.com/EpicGames/UnrealEngine/commit/f9ff3645c2b0fd50ffec64e775eca728826416a4
and
https://github.com/EpicGames/UnrealEngine/commit/cdce7a11e17296aee4967abdac525075b973df07
I did not find any workaround to this and am now trying to build 5.1 from source and hopes it helps.