I had a problem with stereo pairs dropping one speaker when grouped with another stereo pair.
It was really odd - my two pairs are "kitchen" (sonos one's) and "family room" (play 1's + sub). If I was playing music in the kitchen and grouped the family room in, one of the family room speakers would drop off and the only way to get the family room stereo pair working again was to separate them and pair them again.
The exact same thing happened in reverse - playing music in family room, grouped kitchen in and one of kitchen speakers would drop off.... I use Google wifi with three access points.
After a lengthy wait on hold to speak to a tech, he had me send a diagnostic file and immediately identified the problem as one of the kitchen speakers attached to a different wifi AP than the others.
The mesh network uses same SSID on all AP's and seamlessly hands off connections between the three AP's. I've had ZERO issues with any other devices on this network - it's a great solution. To fix the problem I had to unplug on of my AP's (the one that just one of my five speakers was attaching to - let the speakers all attach to the same AP and now everything works (until one of my speakers happens to select a different AP to grab on to and I'll have to chase it down again....
Posting this just to hopefully save someone else from having to call in.