Either the playbase which only has 2.4 disconnects from the network - or one of the surrounds which mesh with the playbase and are not connected to alta drops.
Yes you would suspect dodgy sonos however it does not happen on either aruba or unifi with the same setup.
To the best of my knowledge, they do not have the higher data rate limits that we do by default, which is a pretty good candidate for being the root cause, I could be wrong. But that’s where I’d likely start, with adjusting the minimum data rates for 2.4GHz.
As anecdotal evidence, I had to do this on my own setup with Sonos + Alta despite the bulk of the speakers being well in good range of an AP. Part of this likely stems from the varying data rates. Many modern devices will be 1200mbps+ but speakers don’t need that kind of throughput; makes sense from a BOM perspective, why put a high end wireless card in your speakers if the max data rate they’d need is 300mbps? I digress, let me know how it turns out and/or if you’re still encountering issues.