Using 8.6.6 on windows. I have an app with audio. When I build an app with default First Book and Chapter setting (Features->Navigation) to MAT Intro, the app opens at first launch and gets stuck at first screen with a spinning circle. I found that switching to another app and come back makes the circle go away.
I then build the app with First Book and Chapter set on MAT, 1. This problem goes away.
So it feels like maybe something to do with looking for audio file on intro? (Wild guess)