Third party apps don't appear in the list of available share menu extensions

Question

I have a number of apps installed on my Mac that support being shared to, but when I open the extensions preference pane, only the first party apps are in the list:

I found this command on Apple's support site that looked promising but i tried it and it had no effect.

Does anyone know of another command or troubleshooting step I can try to get this working without reformatting the machine?

I'm on Catalina now but this has been an issue since I first set this laptop up with Mojave when it was new.