"Filename" is a MANDATORY Library field/tag as it is the field/tag MC uses to find each track on the PC where your Media Collection is Stored; i.e. it is the Name the OS also uses to identify that track/file.
Unfortunately, since the "Filename", "Filename (Path)","Filename (name)" and "Volume" Tags/Fields are not Locked it is possible for an Inexperienced user, or a Miscreant, to accidentally, or intentionally, make changes to one or more of those 4 Tags/Fields in either Tag Window which will result in MC no longer being able to locate any Track, or Tracks modified by editing one or more of those 4 tags/fields .
The only Legitimate way to get "Filename" to reflect any changes to any component part of the Filename is to use the Rename, Move and Copy Option.