This is still an issue on AppleTV apps, including Pandora and Spotify. YouTubeTV app works fine and allows sound thru TV/stereo AND the additional AirPlay2 speakers. The only workaround I have found is to use Sonos and skip AppleTV (and its apps) altogether. This allows me to play Pandora playlists throughout our home, including the 5.1 surround system where our TV and AppleTV are located.
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