Application Launcher: adding applications to panel works inconsistently

Steps to reproduce:
- within Application Launcher, right-click on various apps, then select "Add to panel (Widget)"

Expected outcome:
All apps should have their thumbnail appear on the panel.

Actual outcome:
The first three apps I added had their thumbnail appear AOK (Firefox, Dolphin, LibreOffice). When I added Konsole to the panel (and then KSysGuard & others), the panel visibly altered as if something had been added each time, but the thumbnail for each was missing, and the hover text for each reads "Icon" and nothing happens when I click there. I right-clicked on all the panel items I added & deleted them all (including the first three) and then re-added them using the same method as initially, but now none of them are added successfully, and they all create the same blank spot on the panel with the "icon" hover text. See screenshot.

Thanks,
Ken.
***

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: plasma-workspace 4:5.18.3-0ubuntu2
ProcVersionSignature: Ubuntu 5.4.0-21.25-generic 5.4.27
Uname: Linux 5.4.0-21-generic x86_64
ApportVersion: 2.20.11-0ubuntu22
Architecture: amd64
CurrentDesktop: KDE
Date: Sun Apr 5 23:08:08 2020
InstallationDate: Installed on 2020-04-04 (0 days ago)
InstallationMedia: Kubuntu 20.04 LTS "Focal Fossa" - Beta amd64 (20200401)
SourcePackage: plasma-workspace
UpgradeStatus: No upgrade log present (probably fresh install)