That is expected behavior actually. Polls is not a mobile-supported content type so we treat it as if it was an external link. Therefore it opens in a separate browser window and would require authentication directly with the instance.
We don't have near term plans to expand mobile's supported content types to include polls.
I understand you've got to prioritize and there are many, many features I would add to the mobile app before adding polls.
I wonder if there could be something added to make viewing polls on a mobile device less confusing. I can get a DM with a link to a poll, for example, and click on it from the mobile app. It takes me to Safari and I have to sign in. OK, so I do that. And then I want to see more polls, so I click the link I see in Safari to view more polls in that place -- and I get taken back to the mobile app where I can't see any polls at all.
Feels a bit like a practical joke
Anyway to insert any messaging? "The Jive mobile site does not support polls, so you are being re-directed to the full site."