I've been having this issue for a while now and don't seem to find any solution or even what's even causing the problem.
The top panel (wingpanel) works as it should except when I interact with it : when on the desktop, whenever I click on an applet (no matter which) the whole panel just disappear (or rather becomes invisible). It doesn't crash since if I click where one applet is supposed to be, the panel reappears and works as it should (and then either stay or disappear again)
I have no idea what is causing the issue and so far the only fix I have found is to use the shortcut to "show the workspaces" and the wingpanel comes back everytime. I also noticed the issue happens when there are no windows on the desktop, which is even weirder.
I don't know if someone here has a solution, but thanks if you try.