Yes, I stand corrected. Did some more testing and indeed the app does use the phone compass for the orientation indicator. It sort of worked a little better after the .0300 update but I now realize still not accurately.
I also have a Mavic Pro so I tested the phones running a compass app on the MP controller and there was no effect. It's only with the MM controller that the phone compass is affected.
That said, the Go 4 app doesn't have a separate orientation indicator. It's all on the map which uses the aircraft compass and doesn't use the phone compass, just the phone GPS to show the location of the controller. I always just maximize the map to get orientation. This Fly app map works the same way.
My guess is because of the magnetic interference problem with some phones, this feature is going to be removed in a future firmware update. I don't see any way it can be fixed.
But I could be wrong!