HEY APPLE, why won’t my macOS updates, well, update??

So, a not-so-funny thing has happened since updating macOS on my MacBook Pro from Catalina 10.15.3 to 10.15.4:  my updates won't update!! In other words, when I open the App Store, select Updates, then click "Update" next to an app that requires one, a little gray circle sets in motion and spins perpetually.  Even when …