I got it working recently that I could be in an immersive environment in my App with another FaceTime participant while we both see each others personas. The room needed to be shared as it has a presentation baked in to it that one person can present. However, since yesterday whenever I enter the immersive SharePlay experience, the personas no longer appear and only a coin of the other person is visible with the text "immersed" beneath it. The room is still shared, so I can change the slides and the other person can see the changes. If one person exits the immersive space, their persona is rendered from my point of view (in the immersive space) but we can't be in there together.