The “light lines” are optical illusions (related to Mach bands). For example, the pixels on the top-left line are actually no lighter than pixels to the right or below. They just look lighter. It is caused by isolines (lines of constant lightness) having a sudden change of direction.
Illusion or not, the effect is annoying. It is reduced by blurring (so isolines are smoother). I prefer to generate vignetting with a gradient that has smooth isolines (eg ImageMagick “radial-gradient:”).