colorspace filter does not apply dither=fsb when in/out primaries is the same

[Seb]

I have 10bits BT.709 videos and i want to use colorspace filter to convert to 8bits (personal choice over format filter).
The problem is that dither=fsb options does not work when output primaries is the same as input.

This example does not apply any dithering :

% ffmpeg -i input -vf colorspace=all=bt709:format=yuv420p:dither=fsb [...]

But dither=fsb works with this workaround, by forcing a primaries transformation (i used bt2020 but it could be anything).

% ffmpeg -i input -vf zscale=p=2020,colorspace=all=bt709:format=yuv420p:dither=fsb [...] 

Note:Dither is also disabled with fast=1

For now i can use my workaround, but this slows down the encoding by a lot