I’m pretty sure this is due to a hidden thumbnail picture, located high up in the folder hierarchy of your music collection. See How do I remove an incorrect tv show poster - #3 by OttoKerner
Most likely this is due to artists records which are actually a collaboration between artists and are already existent as single entities in the library.
Plex cannot handle these at the moment, so collaboration “artists” like Elton John & Leon Russel are assimilated into the former single artist Elton John.
I only know that the code for the last.fm support is no longer maintained and won’t be getting updates and bugfixes.
In general the Python-based scanners and agents are going to be replaced by native C++ code.
This process started with the new music support and is now in a public preview phase for the Movies media type. New Plex Media Server movie scanner and agent preview
