Or enough other people were fooled and listen to the "fake" Fanny, creating a listener correlation between the real and fake Fannys.
In retrospect, I think it's more likely that Fake Fanny Mendlelssohn did NOT inform Spotify that they were a different person, and Spotify only split it out after a lot of Felix/Fanny fans clicked "I did NOT like this" when the track was put in their Release Radar.