The first one is related to alpha in custom materials, if you have an alpha different than 1.0 when you switch to XR mode everything becomes “broken”.
Sample: Babylon.js Playground
ORIGINALLY I REPORTED TWO BUGS BUT ONE WAS ON MY SIDE, I KEEP THE ISSUE AND SOLUTION FOR FUTURE READERS:
For the second one, I’m not sure if it is a bug or it is intentional, it seems that PostProcess is not used on XR mode:
Sample Babylon.js Playground
EDIT the second one was not a but it was my fault you need to add the PostProcess to each camera https://playground.babylonjs.com/#69UNHV#1
or even cleaner solution
Thank you in advance!