A little while back there was a bug in plasma-org.kde.plasma.desktop-appletsrc where when you dragged a program shortcut to a default panel one had to restart the Plasma shell, logoff, or restart to be able to drag another icon to that panel. I has since been fixed. That said to save time it’s nice to backup plasma-org.kde.plasma.desktop-appletsrc, cause one gets their panels back and the shortcuts they had on them back. What is annoying is the fact that no one thought to include panel size, visibility choice, or width choice. I feel those should be include when backing up and restoring plasma-org.kde.plasma.desktop-appletsrc.
I’m surprised with the very recent bug some haven’t weighed in on this.