FWIW, I just stumbled upon the same issue: have to enter WebXR, then leave, then enter again to get it to go full screen. Using this to pull in the polyfill
<script src='https://cdn.jsdelivr.net/npm/webxr-polyfill@latest/build/webxr-polyfill.js'></script>
<script>
const polyfill = new WebXRPolyfill();
</script>
and then using the modules version
"@babylonjs/core": "^4.2.0-alpha.10",
"@babylonjs/loaders": "^4.2.0-alpha.10",
Here’s a tiny project using npm, etc., you can see the problem. test.zip (1.5 MB) test.zip (1.5 MB)