Created attachment 173214 [details] showcasing the bug In a pdf that has contents enabled when showing them in the sidebar and mouse overing them, sometimes the chapters that popup are scrambled. If you move the mouse from one to another its more visible. Please check attached video.
This is a KWin bug, please start systemsettings, go to the Desktop Effects and disable "Morphin Popups". This should fix it, "Morphin Popups" is gone for the next release of Plasma/KWin so disabling/removing it is the proper fix.