The property can be edited in every spawned instance if for example selected in the scene outliner and edited in the details panel.
However, if I am trying to set an actor in the blueprint default panel, every selection is not being accepted.
The log outputs:
LogUObjectGlobals:Warning: Failed to find object
LogProperty:Warning: Illegal TEXT reference to a private object in external package
I worked around this. I have sub levels. I locked the level where I had the InfiniteAffordableLandscapes Island. I put all the blueprint actors and static meshes, etc on another level.
Still getting complaints about externally referenced issues, but I can assign to the entry on the blueprint.
Your idea about sublevels is the answer for problem when you get “Warning: Illegal TEXT reference to a private object in external package…”. So, blueprint instance and actor to be set as blueprint’s Default should be in one level (Window → levels)