I can't seem to find a fix either. I have messed with adding a .previewLayout to the View in the _Previews: PreviewProvider (as directed here - https://stackoverflow.com/questions/56625931/how-can-i-preview-a-device-in-landscape-mode-in-swiftui) but this just changes the bounds of the screen when in preview, as soon as you click play the orientation returns to vertical.
Would help us save a lot of time! Have you found a good workaround yet?