Despite my detailed explanation, your reply sounds like you didn't fully understand it.
The Apple Music App on Android has no problem casting music directly to any of my Chromecast compatible devices individually.
Android devices have ability to create groups of devices so you can cast audio to more than one device simultaneously. Lets call it a "Chromecast Group."
Most any Android App besides Apple Music also has the Chromecast icon so you can cast audio from that app to any Chromecast Device. I tested several applications, such as Spotify, YouTube Music, etc. and when I click the Chromecast icon I can see a list of all my individual devices and I can see the Chromecast Group listed. When I select the Chromecast group, all the apps I tested cast the audio to all the devices in the group, simultaneously.
On the Apple Music App, if you select an individual device, it casts to that device no problem. But only on the Apple Music app, if you select the Chromecast Group to cast the audio to, it fails to do that like the other apps, and continues to play music on the local device (phone) even though it shows it is connected to the Chromecast group. So this is a bug specific to the Apple Music App for Android since it's not any issue with any other app that casts audio that I tested, even non-Google apps like Spotify.
I'm not surprised other users have not yet chimed in or your developers may not even be aware of the issue because out of all the Apple Music users, only a small portion would both be using Apple Music on Android, AND using the Chromecast Group function to cast to multiple devices at once instead of just one individual Chromecast device. It would be nice if your developers could fix it though. They can reproduce the issue on any Android Device and test with two chromecast devices that have been added to a group using the Google Home application.
There is a temporary work around. This workaround is only required to cast to the group from the Apple Music app as all other apps work fine without this workaround. The workaround is, to load the Google Home app and select the Chromecast Group and choose to cast the audio at the device level, instead of individual app level, so all audio is cast to the group despite what application you are using. Then and only then will the Apple Music App cast audio to the chromecast group of devices.
