The Og Tags are generated wrongly in the Polls module. The image tag will go to the Page Cover photo (which is set through the Studio>Pages>Settings>Cover) instead of the Poll's Cover Photo uploaded to the Poll.
When you try to share the Poll through social media, the cover photo of the page takes precedence over the uploaded cover to the poll. Try to add any poll, and upload a custom cover photo for it, make it public and test the generated link thumbnail here: https://developers.facebook.com/tools/debug/ you will notice that the Page Cover has been selected instead of the Poll's cover photo in the generated thumbnail / link preview. This is usually due to wrong og tags on the page.
I checked my new post with Facebook and Pinterest - both work fine. Then I passed my URL to the FB dev tool - and it was working fine too. Your poll wasn't valid for the FB dev tool because: "Too small image".