Can you check what Trust Level the user is, and the site setting value for poll minimum trust level to create?
I just tried it out, and if the user hasn’t reached the trust level, they should technically not be able to see the “Build Poll” option in the composer. If they paste the markdown for a poll, the topic will still be sent to the review queue, but as you noted, approving it will fail due to the site setting.
I don’t think this is a bug if that’s the case^. Let me know.