I don’t have the same setup, nor access to BBC 6 to test with, but that sounds like a ‘ducking’ issue gone wrong, with the system responding to your voice command partially, and then halfway through the response, changing. In general, I’d try two different things. The first would be to completely remove the Amazon connection to your Sonos system, wait 5 minutes, then reinstall it. In fact, during that five minutes, I’d also be tempted to power off the Beam, wait two minutes, then plug it back in. Once you’ve reinstalled the Amazon ‘system’, I’d test.
If this doesn’t change anything, I’d then be tempted to call Sonos Support to discuss it. You may have to be persistent to get past the Chatbot, but I would want to speak with a person to understand your issue.
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.