Hello there @w14ks !
This issue did pop out a few months ago, and was resolved with a fix from Epic:
The only requirement was for the island to be originally created in Fortnite Creative. If that’s not the case, the operation would not be possible. But if your island was created this way, then the process should work, for which you can submit a ticket at https://www.epicgames.com/help/en-US/