I open an mkv file, go to Tools -> Media Information, and change some text such as Title or Comments, then press "Save Metadata". I close the dialogue, close VLC player, then re-open the same file. The saved information is not there; it didn't save anything.
The most recent version I tried this on was 3.0.4. I've had this problem for years and others have noted it, but it still hasn't been fixed apparently.