How do I get Pixel 4's Motion Sense to work with my music apps? Google's (of course) works. So does Spotify's.
The Pixel 4 support team clearly states that 3rd party devs are the issue, and we should visit https://developer.android.com/guide/topics/sensors/sensors_motion which doesn't seem to have Motion Sense documentation.
I've filed a bug with Google as well. https://issuetracker.google.com/issues/143434555