Media players: all doesn't seem to mean anything

Is there somewhere where I can check the contents of All in Home Assistant’s Media Players list? I can play tracks from My Media on individual devices but, when I select “All”, the track (an MP3 audio file) fails to play on most devices on the list - all of which are online. I’ve used Attic speaker as an individual device within seconds of selecting All and hearing nothing on it, and it works individually but not as part of the group. This seems to apply regardless of whether the service is called from an automation or directly from Developer Tools: Services.

I think, of the list below, only “Corner” actually works when I select All. Is there a way to view or refresh the contents of Media players:All?

I don’t have an ‘all’ so can’t help. Curious though.

Do you have any media player groups?

I have groups only in Google Assistant. Maybe I should just try to list each device rather than using All.