This is because the ScreenPosition node doesn’t take in consideration XR rigs properly. I am trying to achieve the same here Trying to get an HDRP portal to work on VR
I have opened a support issue to get it corrected, but it may take a while.
I’ll let you know when the bug tracker ticket will be created, so be sure to upvote the tracker issue with your vote if you want it to get corrected faster.