That is the intended behavior. More information in this issue at GitHub. The relevant part is:
FYI, on a phone, immersive WebXR modes (both VR and AR) don’t deliver mouse or touch events for screen touches. The immersive view just shows the content of the WebXR-specific opaque framebuffer, and the page’s DOM content (including the scene canvas) is not visible and can’t be touched through the phone screen while the session is in progress. On a desktop, mouse events still get processed for the page content on the 2D monitor, but that’s decoupled from the 3D immersive scene.