I’ve just checked with our engineering team on whether this behaviour is expected or not, and they have confirmed that this isn’t a bug and is by design. In short, if more than one Airplay enabled Sonos player is in a Sonos group, when the group is targeted, the other Airplay enabled Sonos components will drop from the group and these players need to be added back into the group. I hope that clears things up a bit.
Thanks James! This helps a lot. Do we know if there is any way to maintain the sonos groups and prevent the airplay targeting from breaking the existing group? I use Airplay from Apple Music as my primary music source, so it would be a shame if it only ever worked with one speaker 
I’m afraid not. I’ll certainly pass on the feedback to our development teams that this experience isn’t ideal and you’d like to see the groups persist after starting an Airplay session.
One workaround my colleague suggested is when starting the Airplay session, make sure to select both speakers from the Airplay targets list, this should allow you to start the stream on both speakers simultaneously.
Also, Apple Music is available in the Sonos app - so you could always use that to start the music without having to use Airplay.