Had this issue also - at the moment there seems (to me anyway) to be a limit of 7 Sonos zones/endpoints before this issues kicks in. So workaround (hattip to @Mr_Smys who diagnosed it ahead of me) is
- from Settings/Audio disable the Sonos zones until there are 7 or less (you will have to choose which ones you want to hang on to!)
- hopefully you will then see the Ropieee zones appear in the zone selector