Hi @Phihifi
Welcome to the Sonos Community!
I’d first try breaking the stereo pair apart, then pairing them back together. Settings » System » troom name] » Remove stereo pair.
Once you’ve done that, test once more, and if the same issue occurs then please submit a support diagnostic. I recommend you then get in touch with our technical support team who have tools at their disposal that will allow them to give you advice specific to your Sonos system and what it reports.
If you haven’t already done so, I also recommend testing some music service sources on the stereo pair to compare results.
I hope this helps.