Bump. This error persists in 4.22 still and the logged error is still marked unresolved.
In my case, I import new data assets via C++ compilation and consolidate objects to clean up, then fix up references. Due to the compiling via hot reload, blueprint classes that make use of the new, consolidated data assets don’t work unless recompiling manually (no compile error beforehand though) or restarting the editor due to “missing properties”.