My guess as the devices that were participating in SonosNet were restarted as part of the upgrade, STP changed the network topology, causing retries/restart(s) of the upgrade process on the devices that were dependent on the devices that were restarting and/or the system had upgraded successfully but the App wasn’t reporting it immediately due to the STP changes.. Lets see when diagnostic is reviewed.
See the release notes for 15.6 on https://support.sonos.com/en/article/release-notes-for-sonos-s2:
”With this update, we’ve adjusted how some of our older Sonos products are using their internal memory. Because of this change, updates for Play:1, Play:3, Boost, Connect, Connect:Amp, and Sub (Gen 1 and 2) will take longer to complete than before. The reset process for these products will also take more time to complete after this update.”