It seem that Deezer is not going to do anything about it.
I had the same issue last week on my flight back. I could not open the app after the airplane took off, and to my surprise, the music on Favourite Tracks was not available. However, I had the option to SHUFFLE DOWNLOADS and the albums were available.Here is an screenshot I just took which airplane mode activated: On the previous flight I had no issues because I was listening music from before entering the plane. So I guess that if one has the app previously open it does the trick. But honestly, this should not happen ever or otherwise is pointless to pay for a subscription.
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