[x265] Artifacts with vbv-init/vbv-end

[Diego Pasqualin]

Hi, I'm revisiting some parameters in our chunked encoding pipeline and I
stumbled upon vbv-init and vbv-end, which seems to be recommended for the
use case (https://x265.readthedocs.io/en/3.1/cli.html#cmdoption-vbv-init).
However, if I set these two parameters I get really bad artifacts,
strangely more intense in the right bottom part of the output video. I took
this screenshot as example https://pasteboard.co/JxVhD78.png

This is the full ffmpeg command. If I remove the vbv-init/vbv-end it
encodes nicely without artifacts. Any idea on what might be going on? Thank
you very much!

ffmpeg -fflags +genpts -i big-buck-bunny.mp4 -map 0:v -bf 3 -refs 3 -crf 21
-force_key_frames expr:gte(t,n_forced*2) -g 50 -keyint_min 50 -x265-params
"hrd=1:vbv-init=0.5:vbv-end=0.5:vbv-maxrate=1870:vbv-bufsize=3740:keyint=50:min-keyint=50:no-open-gop=1:bframes=3:ref=3:level=40:limit-modes=1:rect=1:amp=1"
-c:v libx265 -flags +cgop -map_chapters -1 -map_metadata -1 -pix_fmt
yuv420p -preset medium -profile:v main -r 25 -vsync cfr -sws_flags bicubic
-filter:v scale=trunc((oh*iw*sar/ih)/2)*2:720 -an -dn -sn -y ./output.mp4
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