When a new thread is saved as a draft, all data including information related to a poll (question, answer, options) is saved within that draft (in the xf_draft.extra_data column). However, when the thread is continued, the poll information is not populated. The revelant code is inside XenForo_ControllerPublic_Forum->actionCreateThread(). The information from extra_data is parsed, but only for thread title, prefix and attachments. So in fact, not only is a poll not restored from the draft, but so is not other information from the extra_data column, including thread status (open, sticky) and watch options.