Used to be able to full screen the League client in 2560 x 1440

I used to be able to fullscreen the League of Legends client in 2560 x 1440 and all of a sudden I am no longer able to. I have no idea what has changed since the last time I have changed it to this setting but it randomly just reset it back to 1920 x 1080 and I haven't been able to even see the option for it anymore. I've recalibrated my monitor settings (my monitor is 2560 x 1440) and I've done the full repair on the client but still nothing has worked.