In Unbounded_Volume mode, when you open swiftUI, exit the app and re-enter the app, only the swiftUI will be displayed, not the unity UI.
You need to track the window event.
https://docs.unity3d.com/Packages/com.unity.polyspatial@1.2/api/Unity.PolySpatial.VolumeCamera.WindowEvent.html
When Background or close event was triggered, you need to close SwiftUI manually.
You can use this until there is a fix: Scene disappears on home button press, SwiftUI persists. How to get scene back? - #4 by berkanozgur