Tried in: Kubuntu 20.04 and 20.10

Issue: When installing applications from Snap, Gnome Software Center, and even Discover which comes built-in with your Kubuntu installation, the apps won't show in the application launcher.

Is this intended behavior?

Testing: It's as simple as installing Postman or whatsdesk from any GUI store, then attempting to run it from the app launcher, or what windows users call the Start Menu.

Sorry If I'm posting in the wrong place. Any help would be appreciated.

When you say app launcher, is that the kckstart enu (Start menu)?
I don't think I have seen that behavior.
Try creating a new account and log in with the new account and check the Start menu.