Guy
Hi folks: Sorry, false alarm! I was quite concerned about this issue because it made Qubes potentially to difficult for me to use because I need a lot of software and files open at once, so being able to see icons is important for me. Turns out the problem resolved itself when I rebooted the system. I've been reluctant to reboot because I had a number of things open I needed to address.
Guy
Whoops, have to take that back. More icons are now showing in the panel, but far from all. Particularly important icons such as for various Libreoffice document types are not showing.
Yup, that certainly sounds like it. I probably should put a note up about various places I'm finding padlocks rather than icons. Thanks!