I don't know what's wrong with Windows Media Player metadata over playlists.
When I trying to relocate some new songs to its new location using notepad++ (So that it could find and play them), Windows media player overwrites on those edits! I din't have this bug before and I haven't done any change on its setting so far!
File Path
column). Do you still see the same behavior if you edit the playlist afterwards? (provided that it hasn't been updated already with the new location by WMP)