To clarify, the app usually (not always) immediately crash when I open another app. It tends to happen when I remain the other app for a few minutes. During that time the sound quality of the music gets noticeably worse. The sound keeps breaking until eventually the app closes.

The app usually does NOT immediately crash when I open another app.

@Kwabz11 It's knowing issue.

Deezer working to fix, but we dont' know how much time it takes

I have the same issue, app on iPhone crashes all the time. Sometimes and often through the week I have to attempt to relaunch the Deezer app 5-8 times before it starts playing music. And I’m on iPhone 13 Pro Max. 
 

not only does the app crash like crazy unlike anything I’ve ever seen, the volume is an issue. Volume skips up and down. I worry my speakers will blow up. I pay for Deezer hifi. This is insane the app on an iPhone is so problematic. All my friends make fun of me for not waiting to Spotify or apples music. I hate those platforms for music but I’m starting to hate Deezer even more for volume and app crashing issue. I have 1TB on my iPhone 13 Pro Max, and have also 300gb. Don’t know if this is a Deezer but that’s I’ve been struggling with for over a year or if this is Apple cause Deezer to crash as a tactic to win Deezer clients, none the less Deezer needs to get to the bottom of it. If this is an Apple tactic causing the app to malfunction. I used Apple Music for a month and transfers all of my music from Deezer to Apple and had no issues with them. I still don’t like Apple’s sound quality which is why I remain with Deezer, Apple has “lossless but it sounds awaful. Deezer is better for our high end sound systems but it crashes and skips volumes.