I have noticed if I have music playing in one room of the house and in the middle of the song I group it to another room the audio sometimes only plays out of one of the stereo speakers even if I see it online, once I power cycle the speaker everything is working as expected. I do have multicasting and all the IGMP proxies setup on my router.
Could this be because there is a lot of information being transferred at once to the new grouped speakers that the multicasting doesn’t keep up? I don’t seem to notice this when I pause the music and group the speakers to then un-pause the music.