With Fullscreen it happens. But with windowed fullscreen it does not, but then the resolution is lower then the native monitor res. You can clearly see then that the picture is not as sharp as it should be.
may try this: set game to fullscreen with 2560x1440.windows desktop must also in 2560x1440. play, then press alt+enter for windowed screen then press alt+enter again for fullscreen. any change ?