So i saw that if you make a copy of your old island in Fortnite Creative, it fixes this issue.
I had this same issue and tried to copy the old island and it worked. Try to do this.
In the creative hub in Fortnite you can ‘Copy’ your island when changing island and everything the original has, will be on the copy one, so you don’t have to rebuild everything to zero. It worked for me, maybe try to do it to resolve your issue.