Ubuntu 18.04: "Add to Favorites" missing for certain qt-based applications?

OS: Ubuntu 18.04 (minimal installation option):

dkb@dkb-ubu1804:~$ cat /etc/X11/default-display-manager
/usr/sbin/gdm3
dkb@dkb-ubu1804:~$ echo $XDG_SESSION_TYPE
x11
dkb@dkb-ubu1804:~$ 

I installed the following qt-based applications using apt install: dolphin, featherpad, gwenview, kate and qterminal.

Out of these, dolphin, gwenview and kate did not show the "Add to Favorites" option when I right-clicked on their icons in the launcher. But featherpad and qterminal had the "Add to Favorites" option. Why is that?

(The situation persists even after a reboot.)

Their .desktop files didn't offer any clues.

• Dolphin

[Desktop Entry]
Name=Dolphin
Exec=dolphin %u
Icon=system-file-manager
Type=Application
X-DocPath=dolphin/index.html
Categories=Qt;KDE;System;FileTools;FileManager;
GenericName=File Manager
Terminal=false
MimeType=inode/directory;
InitialPreference=10
X-DBUS-ServiceName=org.kde.dolphin

• FeatherPad

[Desktop Entry]
Name=FeatherPad
GenericName=Text Editor
Comment=Lightweight Qt5 text editor
Exec=featherpad %F
Icon=featherpad
Terminal=false
Type=Application
MimeType=text/plain;
Categories=Qt;Utility;TextEditor;
X-KDE-StartupNotify=false

• Gwenview

[Desktop Entry]
Name=Gwenview
GenericName=KDE Image Viewer
Comment=A simple image viewer
Exec=gwenview %U
Terminal=false
Icon=gwenview
Type=Application
Categories=Qt;KDE;Graphics;Viewer;Photography;
MimeType=inode/directory;image/gif;image/jpeg;image/png;image/bmp;image/x-eps;image/x-icns;image/x-ico;image/x-portable-bitmap;image/x-portable-graymap;image/x-portable-pixmap;image/x-xbitmap;image/x-xpixmap;image/tiff;image/x-psd;image/x-webp;image/webp;image/x-tga;
X-DocPath=gwenview/index.html
# InitialPreference should be greater than Okular so that Gwenview is the
# primary application associated with images, but less than Konqueror or Dolphin
# so that Gwenview is not the primary applications for folders.
InitialPreference=8
X-DBUS-ServiceName=org.kde.gwenview

• Kate

[Desktop Entry]
GenericName=Advanced Text Editor
Name=Kate
Comment=KDE Advanced Text Editor
MimeType=text/plain;
Exec=kate -b %U
StartupNotify=true
X-KDE-HasTempFileOption=true
Icon=kate
X-DocPath=kate/index.html
Type=Application
Terminal=false
InitialPreference=9
X-DBUS-StartupType=Multi
X-DBUS-ServiceName=org.kde.kate
Categories=Qt;KDE;Utility;TextEditor;

• QTerminal

[Desktop Entry]
Name=QTerminal
Type=Application
GenericName=Terminal emulator

Comment=Terminal emulator

Icon=utilities-terminal
Exec=qterminal
Terminal=false
Categories=Qt;System;TerminalEmulator;
Actions=Dropdown;

[Desktop Action Dropdown]
Name=Drop-down terminal
Exec=qterminal --drop
Icon=utilities-terminal

Name[en_GB]=Drop-down Terminal

Dolphin (shown below), gwenview and kate don't show an "Add to Favorites" option

Featherpad (shown below) and qterminal have the expected "Add to Favorites" option