I had probably installed Joplin manually once.
Have now installed the current Joplin normally after deleting the old, manually installed version as described on 20.04 - How to uninstall Joplin if you don't know how it was installed? - Ask Ubuntu
( I did a reboot after uninstalling before reinstalling the package. )

Joplin is often installed manually, though there are Snap and Flatpak options available. To remove a manually installed Joplin installation, you can follow these steps:

Open Terminal (if it’s not already open)
Remove Joplin:

sudo rm -r ~/.joplin
sudo rm -r ~/.config/joplin-desktop
sudo rm ~/.local/share/applications/appimagekit-joplin.desktop

Enjoy a coffee