I have a similar problem. I’ve been using Logitech Media Server for whole house stereo for a very long time. I’ve decided to give Roon a try, but I cannot get Roon to play grouped AirPlay speakers. I can play speakers individually, but when I group them only one plays.
Equipment in my AirPlay signal path:
Innous Zenith MKII (Roon Core)
MyTek Liberty DAC
RIVA Festival in Family Room (AirPlay)
RIVA Arena in Kitchen (AirPlay)
RIVA Arena in Living Room (AirPlay)
RIVA Arena in Guest Bedroom (AirPlay)
As a side note, I also use two Roon ready devices in my basement that group and play just fine:
HiFiBerry DAC+ (Rec Room)
NAD M12 (Music Listening Room)
The Mytek DAC is directly connected to the Zenith (Roon Core) so it shows in the signal path. In any event, I fixed my problem by using RIVA speaker built-in Google Chromecast protocol, instead of AirPlay.
To make it work, I had to disabled MQA via Settings | Audio | Device Setup | SHOW ADVANCED where I set “Enable MQA Core Decoder” to “No.” All of my RIVA speakers are in a Google Home group where I had to do the same thing. To make them all play, I select the Google Home group as output.