I have a Sonos One that I generally control using Alexa. I have no issues with actually getting Deezer working using this setup; my issue is that playback initiated from Alexa is not reflected in the Deezer iOS app.
For example, if I request that Alexa “Play my flow on Deezer,” the fact that I have initiated a session is not reflected on the Deezer app--whatever I was playing from a previous session initiated from my iPhone would be all that I could see. If I wanted to see what music was playing without having to ask Alexa, I have to open up the Sonos iOS app. If I wanted to add a song to my favorites, or dislike the song or the artist, my only recourse is to use voice commands via Alexa. This is all a bit cumbersome--part of the appeal of Deezer for me is the Flow feature and the ability to fine tune it with a fair bit of granularity, and not being able to do that without shouting at Alexa is not ideal.
Confusingly, things work a bit better going the other way--if I initiate a session on my phone and playback on the speaker via Airplay, some limited control with Alexa is possible (pause, skip, etc.)
Is there any way to enable this currently?