That seems odd, for sure.
Just to confirm, these speakers appear as Left Surround and Right Surround when you look at the settings for the room that contains the Beam, correct?
I’d be tempted to reboot several things, including the Beam and the router.
If this doesn’t resolve the issue, I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it. But don’t post the resulting diagnostic number here, they get sensitive about GDPR.
There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.
When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.
