I have seen a good many posts by people with some Pandora issues (stopping after 1 song or stopping after about 15 minutes).
I know Sonos has been working on some issues. You may want to call them so they have a support ticket for your issue as well. The more people reporting to them the better at fixing bugs.
Only thing I could recommend right now is go into the Alexa app ... go to the Sonos skill and disable it, then re-enable it to pull in most recent code. See if gets any better.
I believe we about to see a new firmware release - so maybe it will help resolve. After the next firmware update I would probably go in and do the Alexa app skill disable and re-enable again after that too.
Of course using the controller and starting Pandora would be the other work around.
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