bandings when baking very soft shadow

I was using very high quality and very high res lightmap to bake the stationary light.

but there are some bandings at the edge of the soft shadow.

I thought it was because the lightmap is 8 bit,and lacks of information when the lightness is very similar.

Is it possible to bake a 16bit lightmap or using the dither technique to solve this problem?

Or is there any other settings to take up the quality of ‘direct lighting shadow’ ?