If you have the backlog enabled in a next-gen board and there are issues there when you disable the backlog, all the issues are moved to the board but if you re-enable the backlog, the issues will remain on the board and you lose the information about issues you had in the backlog. There's no way to recover your backlog.

As a Jira Software user, I want the issues that used to be in the backlog when it was active to be moved back to it when re-enabling the backlog.

Also, the user should be notified that the issues will be moved to the board when disabling the backlog in next-gen projects.