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".

Tell me a bigger red flag to identify junk apps. Every "homemade" low cost app has similar inexplicable bugs that they don't care to fix.

My bank app forwards me to the settings every time I try to send a bank wire because I never allowed access to contacts.

Apple doesn’t care to fix it either. Why can an app repeatedly turn off the podcast I’m listening to yet I have no recourse to stop it?

I have to keep pressing Play on my airpods. If I’m not using airpods it can be impossible to resume my own audio with the app open.

Audio apps have greater permissions to run the background, right? Wonder if it’s related to that.

Same with the Technogym app on Android. Every time I connect to a new piece of equipment my audio pauses. Infuriating!