Windowed mode resolution issues

I play FFXIV at 1600x900 resolution windowed mode. I could maximize the window by clicking the box on the top right corner to fill my screen (so the graphics looked a bit stretched) and it would stay at 1600x900 but not anymore.

Now when I maximize the window, it switches to 1920x1001 (my desktop resolution is 1920x1080). Resizing the window manually didn't help either. Neither did borderless windowed mode.

It's like my resolution was "locked" before but not anymore. Anyone know how to keep it locked again?

Have you tried running in full screen mode with a 1600x900 resolution?