When i used my Firestick 4K for Tidal Atmos and this problem arose, I tried both restarting and disconnecting,but neither worked.I asked about it here on QQ (after trying Tidal support ) and got the answer that I had to uninstall/reinstall and that worked and has ever since.But I couldn't risk my speakers anymore so I quit Tidal.Several times the the Atmos went away in the middle of an playlist/album.A little silence and then bang, +20db non Atmos music.The other main problem is when I played an playlist and it ended without me noticing, then Tidal decides to play a non Atmos song becouse they think I'll like it, even if I have only Atmos music in my playlists.Amazon Fire TV 4K Tidal App needs to see an Atmos capable system via its HDMI.
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When it happens that he don't see it, for whatever reason, a new HDMI handshake and/or Tidal App handshake has to be launched.
This may be done by rebooting, better disconnecting the Fire TV. Reinstalling the app also works.
If they fix this problems I will be back, but I don't really suppose that will happen.