If Sonos doesn’t post release notes with an update, it is usually just “bug fixes and performance enhancements”.

And Sonos doesn’t release details on these “bug fixes and performance enhancements”.

From Sonos about updates without release notes:

“...the reason why certain versions are skipped is that we only post release notes for updates with customer-facing features.”

If there is no update information shown here at this link:

https://support.sonos.com/s/article/3521

… then it’s usually minor bug fixes, or perhaps security updates …and not any additional functionality, or anything most of us would actually notice, at least that’s the ‘rule of thumb’ that I’ve come to understand with such updates.

Which, in fact, I find extremely frustrating. 

Exactly, you never know if your particular (possible) issue has been addressed.

Pretty useless way of doing things really for the end user. Most (not all) companies give a breakdown of what has been improved or fixed.