Arbitrary icons missing
Running on Arch Linux, gnome-shell 44.4-1, Wayland. Some applications have icons, some don't:
I don't see any reason why Tweaks (and other applications) would not have any icon. It's defined in the same way Text Editor's icon is, and it's located in the same directory.
$ grep -F 'Icon=' /usr/share/applications/org.gnome.TextEditor.desktop
Icon=org.gnome.TextEditor
$ find /usr/share/icons -iname 'org.gnome.TextEditor*'
/usr/share/icons/hicolor/scalable/apps/org.gnome.TextEditor.svg
/usr/share/icons/hicolor/symbolic/apps/org.gnome.TextEditor-symbolic.svg
$ grep -F 'Icon=' /usr/share/applications/org.gnome.tweaks.desktop
Icon=org.gnome.tweaks
$ find /usr/share/icons -iname 'org.gnome.tweaks*'
/usr/share/icons/hicolor/scalable/apps/org.gnome.tweaks.svg
/usr/share/icons/hicolor/symbolic/apps/org.gnome.tweaks-symbolic.svg