I have run into the same issue as this ticket. https://community.atlassian.com/t5/Jira-Software-questions/User-stories-disappeared-from-the-backlog/qaq-p/1106330
I have migrated the stories from with next-gen board into the classic board and not able to see any of the stories in the backlog. In fact, my board currently shows the Epic from the Next-Gen Board attached to the stories that were migrated.
