Not a bug, the scale doesn't automatically change because some people want to tweak it and automatically setting that according to fov would counteract their settings.

Either enable automatic fov adjustment in the config or make it recalculate fov once by typing /helmet for fov in the chat.

Or you can set the vignette's scale to whatever you want :P You can even set it outside of the field of view so you don't have a vignette at all.

ok thank