Given a setup where to screens are used next to each other, and the Plasma panel is set in between these two. In such case maximized windows will always go below the Panel, the option "Always visible" simply does nothing due to the bug #94470 (also see #167852 and others). It would be nice if that option would not be shown for panels which are positioned in between screens where the option does not work anyway.
*** This bug has been marked as a duplicate of bug 323230 ***