MimeType not changeable

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1

I can add my mimetype-list in ~/.config/mimetype.list

[Added Associations]

audio/mpeg=vlc.desktop
video/mp4=vlc.desktop
application/pdf=org.gnome.Evince.desktop
...

or via NixOS-HomeManager-declaration:

  xdg.mimeApps = {
    enable = true;
    defaultApplications = {
      "text/plain" = "org.gnome.gedit.desktop";
      "text/markdown" = "org.gnome.gedit.desktop";
      "text/x-go" = "codium.desktop";
...

Adding/changing a filetype via Gnome does only work with the ~/.config-version. The 2nd attempt throws an error.

What do I have to change?