Bug: ATV4K won't play music on BOTH the TV and HomePods in another room

TLDR Summary: The problem appears to exist only within the Pandora app for Apple TV (Pandora's been my go-to music service for the last 12 years with many personally-perfected "stations").

Longwinded process story: I contacted Apple Support and worked with a guy on the phone who dutifully asked me to power-down and restart everything (ATV4K, HomePods, WiFi hub/router, etc.). I dutifully complied. When the ATV4K came back on with an Internet connection, I inadvertently selected the first item on the screen which happened to be Netflix's "Moving Art" (great UHD/4K nature footage with innocuous "waterfall music" that I normally mute). With the audio playing rather loudly through the soundbar, I brought up the control center airplay screen and selected the HomePods and... it worked! Relaxing music playing via the soundbar and HomePod Mini stereo pair simultaneously!

I felt like an idiot and I racked my brains knowing that I've rebooted everything many times since I first encountered this problem 9 months ago! Given this, the very patient Senior Apple Support dude and I were both still interested in WHY it finally worked this time. It occurred to me that while I've been off and on tvOS BETA releases while experiencing this issue, I may not have actually powered down since the last time I turned off the settings option to accept BETA updates in recent days (only a few Settings --> System --> Restart). We decided to turn the BETA option back ON and test again - I even tried before rebooting and as is my habit, this time I tested using Pandora... sure enough, no soundbar audio with AirPlay HomePods also selected! But simply switching the BETA option as the cause didn't make sense. I noted that ATV distinguishes between audio-only apps and video apps (the former continues playing while surfing around other apps until something else is actually played). Back to NetFlix and the sound played everywhere it should. To verify it's not an issue with all audio apps, I loaded (Apple's) Podcasts app, and again the audio played everywhere it should. Back to Pandora and no local soundbar audio with an AirPlay speaker also selected.

It had never occurred to me that such a thing would be app-specific, but the Apple TV Pandora app sucks in many other ways... It lacks many features found in their current iOS and Android apps, so I'm assuming its underlying code is too old play nice with all the latest Apple TV AirPlay tricks.

Final summary: this IS a bug, but only within the Pandora app for Apple TV -- as of this writing: v1911.1 (released Nov 20, 2019). I'll tell them about it and cross my fingers as I consider finally weaning myself from their service for a variety of other annoyances... Hmmm - Apple Music or Spotify?

(bump - original post clearly a TLDR thing)

I'm really curious - can anyone else please try to duplicate this problem on Apple TV 4K and a set of HomePods? It's an easy test: simply configure ATV4K to play locally (on the TV/Receiver) AND on HomePods via AirPlay simultaneously.

For me, the sound plays on the HomePods but stops on the TV/Receiver even though the ATV screen implies it's playing on both.

For that matter, I also see the problem when I try to play on ATV and the only other AirPlay speakers on my network (my MacBook). Oddly, I can play it on both the MacBook and the HomePods simultaneously, but the TV/Receiver still goes mute until I turn off the AirPlay connection(s).

Thanks for the effort and the info, @vazandrew!

Weird. This hasn't worked for me since I bought the ATV4K last march. Perhaps it's because of the way my ATV4K's HDMI output passes through my (Sony HT-Z9F) soundbar which is then connected to the (LG) TV's ARC HDMI... though this is like most home theater configurations with an AV receiver. Everything else works great (Dolby Atmos on the soundbar; HDR video on the TV, etc.).

To be clear - you initiated this from the Apple TV 4K? I can successfully initiate music from any other device (such as my iPad Pro) and play it on multiple speakers including the ATV4K/TV/Soundbar & HomePod Minis :-). Just can't play ATV4K audio locally+AirPlay elsewhere.

Thanks again!

Despite my speculation I think tried bypassing the soundbar before... I just did it again. Same behavior: the Apple TV's local HDMI audio output stops when it also sends audio to an AirPlay speaker.

With your confirmation that this isn't a universal problem, I'm now strongly suspecting an ATV4K hardware issue.

Oh yeah. I'm planning on contacting Apple Support about it - I had actually logged the issue via Feedback Assistant last April (such a specific & lonely issue just didn't feel like a hardware issue - still doesn't but what else could it be?).

To be honest, the discovery that the issue was app-related only occurred by accident when I inadvertently chose a video app to test audio playback instead of Pandora (normally I only want to use the feature when listening to music). Nonetheless, your recommendation to call on Apple Support was spot-on.

While I don't monitor Apples's developer tools, I suspect that Pandora engineers could likely remedy this bug by just recompiling with Apple's latest & greatest libraries and minimal code changes (if any). Clearly they figure most Apple TV owners grew up on iTunes and are now playing Apple Music.

My main gripe about Pandora (apart from their neglecting certain flavors of their app) is that they're rudely anal-retentive about a valid account holder running more than one instance of Pandora at a time - even if only one instance is actually playing and both are using the same WiFi/Internet gateway. Example: Their otherwise perfectly good HomePod implementation is playing music and I decide to open the iPhone app to modify a station - after a little while the music is interrupted with a cheerful voice telling me that another person is using my account and forcing me to take action to resume playing music (basically I need to tell Siri to resume playing music). Grrr. ;-|