Oh, Apple…
We had an issue similar to yours here - VideoTexture not rendered to in Apple Vision Pro . It might not be entirely related, as you are not using an external player, but it does seem like the behavior is very similar.
I am not sure as to why it doesn’t work in WebXR but works outside. Checking the quest - it does seem to be working correctly there. I took the liberty to generate a playground - Babylon.js Playground (babylonjs.com) to be able to debug this as well. I can try debugging it on the vision emulator (sorry, I don’t have a device yet, long story), but i can’t promise a proper solution. I hope it’s something trivial that we can fix on our side.