Two possibilities spring to mind:

1. They will have added code that declares the app requires an exclusive audio context. So iOS pauses all other audio when the app is foregrounded.

Or

2. It’s possible that they use anti screenshot technology which sometimes involves embedding a secure video in place of an image. The video playback might be grabbing the audio context.

I've had this a few times on Android (eg the new Subway app). I'm 99% sure it's the latter but not for security, just a fancy splash screen animation that was implemented as a video without thinking about setting it as "no audio".