Chroma bleeding is actually really weird, it shouldn't really happen and I think I know why it's happening to you!
Your input is an AVS Script from what I see, which handles 4:2:0 Type 1 but not 4:2:0 Type2!
So, the chroma location is wrong, hence the "bleeding".
Can you try with:
Code:
ffmpeg.exe -i "AVS Script.avs" -vf scale=out_color_matrix=bt2020nc:out_h_chr_pos=0:out_v_chr_pos=0 -pix_fmt yuv420p16le -strict -1 -an -f yuv4mpegpipe - | x265.exe --y4m - --dither
followed by the rest of your x265 command line? This should solve the chroma bleeding problem.