There seem to be a lot of subtle conditions that can trigger this. Let's collect them all here in one place.
Here's one that I notice. X11, Neon, Nvidia prop drivers. I have a few dodge toolbars (bottom and right). If plasmashell crashes (which it does) and restarts, the panels no longer dodge. I _think_ this also happens with hiding behavior. The only workaround I've found is to enter edit mode and then exit (don't have to do anything). Then they dutifully pop down like they are supposed to. I think some of the "strange" behavior I've seen is where plasmashell crashes while I'm away, restarts itself and then the panels are unable to hide again but I can't prove that part.
Please open a new bug report for that and mark it as blocking this one. Thanks.