I am relatively new to unreal and was just undocking some windows and ran into this same issue. (Specifically with the world outliner, not the others as far as i can tell.) You can see by the attached screen shot that my World Outliner shows up in the list of window options in my windows taskbar, but clicking on it just puts focus back onto the editor, not re-opening the outliner as expected.
I have also tried deleting the “Saved” folder from the project folder and still no success. Also, for what it’s worth, When I initially tried to undock the world outliner, I was trying to drop it onto a 4k monitor which was NOT the main monitor that i had the editor up and running in. This problem persists into making a new map as well.
WORKAROUND FOUND - If you right click on the world outliner in your task bar, then “MAXIMIZE” it with the RMB context/options, that seemed to put the window at full screen, at which point i was able to move it and dock it in the desired position again. Hope this helps.