Hello. Did you activate Deezer’s offline mode after setting the phone to airplane mode. 
That might be normal behaviour as Deezer will only play downloaded songs in offline mode.
If you can provide a video with the problem it would be really helpful.
Thanks

Queue:

Downloaded song

Downloaded song

Song added to through 'add to queue' but not downloaded

Downloaded song

Downloaded song

When going into Aeroplane mode after, the playback will simply be interrupted because this bit of the Deezer app has a bug, or is not programmed with UX fully thought through.

Speaking of UX further:

"Did you activate Deezer’s offline mode after setting the phone to airplane mode. " 

Of course I did not. In 2022 a user ought not be expected to stop what it is they are doing to then open the app, go into settings, and enable Offline Mode manually.

A music playing app should be optimised for as little few active user interactions for it so what the user expects.

Thanks for all the info provided and also for your honest feedback (I’ll make sure to pass your feedback to our devs). And you are right, it does activate offline mode immediately when you switch to airplane mode.
Keep in mind that only downloaded songs will play in offline mode. Once you switch to airplane mode you should click on favourites>downloaded music>play your music
Let me know if that works