You found it, phred! I renamed mb_LyricsReloaded.dll (in my case version 1.2.2) in my Plugins directory and the "_source.log" creation ceased. So, the log is still happening in the latest version.
Until Mayibongwe can fix this, I'm rolling back to 1.2.0.8. The only "loss" there is the ticket ad crop search, and the addition of two broken Musixmatch entries, easy enough to un-check. An alternative is to add "_source.log" to my MusicBee cache deletion/cleanup batch file I run nightly. Maybe I'll just do that, Mayibongwe is usually a pretty quick fix.