libx265: -bufsize and -maxrate does not complement -x265-params vbv-init=0.9

[Boopboop]

Summary of the bug: specifying the general -bufsize and -maxrate options, instead of using -x265-params vbv-bufsize=:vbv-maxrate=, forces vbv-init=0.75, instead of working together with a -x265-params vbv-init=0.9, or leaving it out and letting it default to x265's default 0.9.

How to reproduce:

% ffmpeg -report -loglevel 99 -to 1 -i DSC_0021.MOV -an -c:v libx265 -profile:v main10 -level:v 4.1 -bufsize 20M -maxrate 20M -preset:v veryslow  -x265-params pools=6:frame-threads=1:vbv-init=0.9 DSC_0021.mp4
ffmpeg version N-120196-g6b028859d9 Copyright (c) 2000-2025 the FFmpeg developers
  built with Apple clang version 17.0.0 (clang-1700.0.13.5)
  configuration: --prefix=/opt/homebrew/Cellar/ffmpeg/HEAD-6b02885_3 --enable-shared --enable-pthreads --enable-version3 --cc=clang --host-cflags= --host-ldflags='-Wl,-ld_classic' --enable-ffplay --enable-gnutls --enable-gpl --enable-libaom --enable-libaribb24 --enable-libbluray --enable-libdav1d --enable-libharfbuzz --enable-libjxl --enable-libmp3lame --enable-libopus --enable-librav1e --enable-librist --enable-librubberband --enable-libsnappy --enable-libsrt --enable-libssh --enable-libsvtav1 --enable-libtesseract --enable-libtheora --enable-libvidstab --enable-libvmaf --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libxvid --enable-lzma --enable-libfontconfig --enable-libfreetype --enable-frei0r --enable-libass --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libspeex --enable-libsoxr --enable-libzmq --enable-libzimg --disable-libjack --disable-indev=jack --enable-videotoolbox --enable-audiotoolbox --enable-neon
  libavutil      60.  4.101 / 60.  4.101
  libavcodec     62.  5.100 / 62.  5.100
  libavformat    62.  1.101 / 62.  1.101
  libavdevice    62.  0.100 / 62.  0.100
  libavfilter    11.  1.100 / 11.  1.100
  libswscale      9.  0.100 /  9.  0.100
  libswresample   6.  0.100 /  6.  0.100

(snip)
x265 [info]: HEVC encoder version 4.1+1-1d117be
x265 [info]: build info [Mac OS X][clang 16.0.0][64 bit] 10bit
x265 [info]: using cpu capabilities: NEON Neon_DotProd Neon_I8MM
x265 [info]: Main 10 profile, Level-4 (High tier)
x265 [info]: Thread pool created using 6 threads
x265 [info]: Slices                              : 1
x265 [info]: frame threads / pool features       : 1 / wpp(17 rows)
x265 [info]: Coding QT: max CU size, min CU size : 64 / 8
x265 [info]: Residual QT: max TU size, max depth : 32 / 3 inter / 3 intra
x265 [info]: ME / range / subpel / merge         : star / 57 / 4 / 5
x265 [info]: Keyframe min / max / scenecut / bias  : 25 / 250 / 40 / 5.00 
x265 [info]: Lookahead / bframes / badapt        : 40 / 8 / 2
x265 [info]: b-pyramid / weightp / weightb       : 1 / 1 / 1
x265 [info]: References / ref-limit  cu / depth  : 5 / off / off
x265 [info]: AQ: mode / str / qg-size / cu-tree  : 2 / 1.0 / 32 / 1
x265 [info]: Rate Control / qCompress            : CRF-28.0 / 0.60
x265 [info]: VBV/HRD buffer / max-rate / init    : 20000 / 20000 / 0.750    <= Here, init 0.75 instead of 0.9
x265 [info]: tools: rect amp rd=6 psy-rd=2.00 rdoq=2 psy-rdoq=1.00 rskip mode=1
x265 [info]: tools: signhide tmvp b-intra strong-intra-smoothing deblock sao
x265 [info]: tools: dhdr10-info

[Boopboop]

ffmpeg report