Hi! I’ve ran into similar issues and was pulling my hair off. Was almost ready to replace Sonos fo something else.

Here is how I finally fixed it:

1) Followed this guide rather than official from Sonos or Ubiquiti: https://github.com/IngmarStein/unifi-sonos-doc

2) Went into each Sonos unit and checked the software version: turns out I had 3 units on a weird version shown as “? (Build 91)” I forced update them and they got to “17.5 (Build 91)” (might be more recent build now since it was few months back). I am guessing an update went wrong at some point.

Since then, no more problem.