When having a maximized window (in my case AUR Firefox), then switching from "auto-hide + Floating" into "auto-hide + Not floating", the windows are retracted/cropped out on the place where the panel would be (in my case, left corner). Obs: "plasmashell[...] Use this for bugs in Plasma 5's shell and applets." is outdated since plasma 6's launch.
Created attachment 169125 [details] Issue
The maximized window has to be minimized for the issue to happen.
Seems like it's fixed in version 6.1