Since I’ve found another person with the same problem as your, so I’m gonna paste it down here…
I think you need to duplicate your map to solve this problem. Recently, I also experienced the same problem, so I duplicated my map and the problem was solved. This might be related with your map file being corrupted or something else.
But that also means you won’t be able to update your original map with the same map code if you already published it because you only duplicate the map on the new project. (This is only a theory, but I think it’s make sense).
Anyway here the link for tutorial how to duplicate the map…
How to duplicate or copy a UEFN Project Easily! #uefn #fortnite (youtube.com)
Hope this help you…