If a filename/commonsMedia (and possibly other types) is added as a the value for a property but the initial letter is lower case then this is displayed as upper case in the interface but stored as lower case in the backend. Since the diff displays the value as shown, rather than as stored, the result is a diff which looks like it should have been a null edit.
As an example this diff changes fr-Japon.ogg to Fr-Japon.ogg but the diff makes it look like a null edit.
- Either have the diff display the stored value or
- Capitalise first letter during initial storage (I'm guessing something similar is done to underscores already)