I have tried uninstalling an reinstalling both apps, also disconnecting the Spotify service from the Sons app and reconnecting it.
Best answer by SebyM
Looks similar to this thread (https://en.community.sonos.com/troubleshooting-228999/spotify-app-not-showing-sonos-6762270/index4.html), but that one's closed with no apparent solution give (rather unprofessional...)
Every network is different, and there is no single fix for issues like this. Please try the above-mentioned troubleshooting steps, and submit a diagnostic report if any issues continue. Reply here with the confirmation number and I'll take a look.
This is the 2nd time I'm running into this issue. Last time I didn't bother with it too much but now it annoyed me as I was just showing off my Sonos to a friend.
Rebooted the Sonos 1's, no fix. Noted that the Sonos pair did not show on the Android app either and the Android Spotify client did not show on either the Windows or Chrome apps (as targets).
Went with my next immediate guess and rebooted the PC. It fixed the issue on PC.
Then I moved to my Android phone. Same issue there. Disabled Wifi and moved to LTE, no change. Did not reboot but swiped away the Spotify App and reopened it, it fixed the issue on Android and both the Sonos and Android Spotify player (target) showed up on the Chrome App.
I think the Spotify app is the culprit somehow; it's the only thing that makes sense to me.
Cheers and have a great evening, ask away if you need more details,