[DaveBlake]

My first guess would be that the music files of your "home made compilations" have Muiscbrainz Id tags, that is the only reason exitsing library entries would get modified. If mbid tags exists Kodi assumes they are accurate and definitive. You say "all other tags are left blank", but are you sure? In Mp3Tag select a song and use ctrl+T to see the extended tag list, that will show more tags than the usual main screen does.

But I also want to know more about the "home made compilations". If you have these songs on other albums in your library, do you in fact just want to make playlists of selected songs instead?

Also to help Kodi know something is a compilation set the COMPILATION tag in each music file.