i think its a visual bug, try to select or refresh

Dss0 above 2048x1536 the game don't want to set a higher resolution in fullscreen mode. in console with the command r_mode above 2048x1536 it reset to default 800x600 after vid restart. the only way to have the max resolution of the screen its in windowed fullscreen but the greyed resolution isn't the one who is set, i think its the max screen resolution when i compare the screenshots above