We have experienced this issue using an AppleTV 4K, running tvOS 14.3 and Pandora version 1911.1. The desired use case is to have sync'd Pandora between the AppleTV "TV Speaker" audio (HDMI output to a Denon Receiver and then distributed to a 7.1 system) and AirPlay 2 to a stereo pair of HomePods (the originals, not the minis). Interestingly, the reverse setup (Pandora on HomePod and AirPlay to the AppleTV) does work.
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