"open with" default app not working

Mac OS 10.14.3 [Mojave]: QuickTime Player is set as the default app for Finder's "open with" for .mp4 files on my Mac. This is what I want. But when I double-click an mp4 file, the kid3 app [a tag editor] launches instead of Quicktime Player!? Why and how is kid3 invisibly hijacking the default app for "open with" for mp4 files? I found nothing related to this in kid3's preferences.