Hi @dennisbp, welcome to the Sonos Community!
With voice assistants, generally all commands work or no commands work, so it’s definitely strange that only stopping music doesn’t work. If you visit your Google Activity after asking Google Assistant to pause music playback, does the voice command show in there?
I would recommend unlinking Google Assistant from your account, then re-adding it. You can unlink Sonos from Google Assistant by opening Google Home and going to Settings » Works with Google » Sonos » Unlink Account, or by opening the Google Assistant app and selecting [User Icon] » Settings » Home Control » Sonos » Unlink Account.
If the above doesn’t resolve the issue, then start playback and give the command to stop music. After you receive the error from Google Assistant that no music is playing, I’d suggest submitting a diagnostics and reaching out to our support team for further assistance.
I hope this helps!