MP4 Vs MKV (File.Modified)

[Darius1968]

Ordinarily, when I update a video file's TITLE metadata tag, the file.updated variable and the corresponding XMP metadata tag for date modified are both updated.  That is... If I update an MP4 file.  However, for the case of an MKV file, when I do the same thing, the XMP tag is still updated, but file.modified isn't.  Why is this?  file.updated is updated to the latest date of modification for both MP4 and MKV files. 

[Mario]

Metadata goes into the sidecar XMP file. No change to the video file.