Yeah has been happening since UEFN started, but they are only Warnings, not Errors.
They do not effect the ability to publish islands.
All islands created with UEFN using stock fortnite props gets these warnings
When using Build Landscape, just ignore the Yellow Warnings and filter them out with the drop down menu on the bottom of the results window.
Look for the Red Errors, they matter.
It’s because UEFN uses a later UE and the majority of Fortnite Props were created in an older UE,
I guess it’s easier to ignore the warnings, than to fix the ton of content.