I think it's because you didn't adjust primaries;
zimg (avsresize) works ok, and you get similar artifact if you don't adjust primaries
Code:
ColorBarsHD(9600,1000, pixel_type="YUV444P16")
Crop(0,0,9600,500)
AddBorders(100,100,100,100)
z_convertformat(pixel_type="rgbps", colorspace_op="709:709:709:l=>rgb:linear:709:f")
GaussResize(width/10,height/3, p=2)
z_convertformat(pixel_type="yuv444p16", colorspace_op="rgb:linear:709:f=>2020ncl:st2084:2020:l")
#don't adjust primaries for 2020
#z_convertformat(pixel_type="yuv444p16", colorspace_op="rgb:linear:709:f=>2020ncl:st2084:709:l")
#tweak(sat=0)
#convert to 8bit srgb preview
z_convertformat(pixel_type="rgbp", colorspace_op="2020ncl:st2084:2020:l=>rgb:srgb:709:f")
#convert to 8bit srgb preview 709 primaries
#z_convertformat(pixel_type="rgbp", colorspace_op="2020ncl:st2084:709:l=>rgb:srgb:709:f")