In Kubuntu 20.04, after installing updates via Discover, if I try to navigate applications in left pane, upon clicking each application-category, it says "Sorry, nothing found...". I easily resolved this by closing Discover after the update, and then I was able to see the apps by clicking on the categories. This is minor issue, but I thought I should report it. I'm using Discover 5.18.5, so maybe you've already addressed this issue 15.19.0. I wouldn't know. To reproduce, you'll need a to test a system that actually needs updates, and just install those update using Discover and then try to navigate the Application categories. Merely checking for updates doesn't cause this issue; it only happens after installing update using Discover. You'll find that you have to close and reopen discover before you'll be able to navigate those application categories.
Yes, this was fixed in a later version. Thanks for reporting, though.