Yesterday, I went from kde-plasma/plasma-desktop-5.5.5-r1 to -5.6.5, after which I am not seeing nearly as many icons as I used to. A lot of other KDE packages were upgraded as well, and I don't know where to start with troubleshooting.
In Dolphin, I no longer see icons for flac or mp3 files, just blank space where an icon should be. I haven't checked other file types. I have a panel with several Quicklaunch widgets. There, I see icons for many of the launchers, but not others -- I just see blank space where they should be. Clicking the blank space does launch the program. Normally, the dialog brought up by 'edit launcher' shows the icon for its .desktop file and lets me click on the icon to change it. Now, the dialog does show the icon, but it's not clickable so it's not editable.