[nulld3v]

Are you using a non-default theme or are you using custom KWin scripts (e.g. to enable "window gaps")? Both my laptop & desktop run near-default KDE and moving the mouse to the top-right + clicking always closes the maximized window.

[stavros]

Hm no, I just run the default. It's good to know I did something wrong, I'll look into it. Thank you!