The “Level” window doesn’t work with world partition unfortunately.
The good news is that there are these two folders inside the content folder:
They contain every file used by “OneFilePerActor”, so you can just copy the InstancedFoliageActors across the folders.
If you want to copy just the correct files you can get the name with this option on the right click:
It should return something like this:
C:/YourProject/Content/__ExternalActors__/Maps/MapName/MapName/5/F6/XYCNN57880XXECQHQSF9ST.uasset
I’m glad I found this workaround because I spent many hours painting the foliage and I didn’t want to waste them 