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've created this test Poll on UNA: https://una.io/page/view-poll?id=71 when I post this link to the debugger it doesn't show any photos at all.