(Somehow I wasn't able to find any solution by googling, so I'm asking here.)
Win+Shift+Right / Left Arrow used to work correctly for me for a long time to swap my window to my second monitor quickly, but since a few months ago it started stretching the window across both monitors, making the shortcut useless for me. I can still manually drag a window with the mouse, but for my workflow I need to be able to switch back and forth fast. The problem happens with any application that has a maximized window.
I have a laptop with Windows 10 and a XP-Pen drawing tablet as my second monitor. Disconnecting and reconnecting the monitor or trying other monitors still keeps the problem.