WebXR browser API that Shapespark uses requires user gesture to enter the VR mode. A gesture is a button click or a key press, the API doesn’t allow websites to enter VR mode automatically.
Does the IPhone problem happen for a scene hosted on Shapespark server or self-hosted? Recently we added a workaround for a new bug in Safari that prevents entering a VR mode, but the fix is not yet pushed to the desktop app, so self-hosted scenes are still affected by the bug (see this post).