Created attachment 676965 [details]
Before hit apply

When adding File Associations and Application Preference, KDE/Plasma forgets
Application immediately on "Apply".

The attachment shows how LabView is listed in the Application Preference, as
soon as I hit "Apply", it disappears.

The irony is, it seems to have been recorded, here's my mimeapps.list:
    [Added Associations]
    application/LabView=LabView.desktop;

    [Default Applications]
    application/LabView=LabView.desktop;

and here is LabView.desktop:

    [Desktop Entry]
    Comment=
    Exec=/usr/local/lv71/labview
    GenericName=LabView Development
    Icon=/usr/local/lv71/linux/icons/labview-3d-big.xpm
    Name=LabView
    NoDisplay=false
    Path[$e]=
    StartupNotify=true
    Terminal=0
    TerminalOptions=
    Type=Application
    X-KDE-SubstituteUID=false
    X-KDE-Username=

BTW:  I added the indents.
I already checked out a similar problem on the interwebs, I already have a
symbolic link (and it doesn't make a difference one way or the other):
   lrwxrwxrwx 1 danny users 51 May 14 09:51 .config/mimeapps.list ->
/home/danny/.local/share/applications/mimeapps.list