Created attachment 108364 [details] Screenshot of the bug Replicate by opening dolphin, and right-clicking on the display area towards the right side of the screen. The sub-menu will show up on the top right.
Bug in Qt. Supposed to be fixed in 5.9.2. This is the same as the plasmashell context menus you reported.
Really misplaced sub-menus are not fixed in qt 5.9.2. Sometimes "open with" sub-menu from dolphin context menu opens in wrong position too.