ffmpeg: How to keep vfr unchanged during reencode

Question

I have a file with variable frame rate that I would like to reencode. My attempts so far always lead to the frame rate being changed from 23.98 to 24. I have tried "-vsync vfr" and "-vsync passthrough", but to no avail.

input file (ffprobe):

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'snippet.mp4':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: mp42isomhvc1
    creation_time   : 2021-05-30T14:23:59.000000Z
  Duration: 00:54:23.92, start: 0.000000, bitrate: 11430 kb/s
    Stream #0:0(und): Video: hevc (Main) (hvc1 / 0x31637668), yuv420p(tv, bt709), 1920x1080 [SAR 1:1 DAR 16:9], 9269 kb/s, 23.98 fps, 24 tbr, 12288 tbn, 24 tbc (default)
    Metadata:
      creation_time   : 2021-05-30T14:23:59.000000Z
      handler_name    : Video Media Handler
      encoder         : HEVC Coding
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, 5.1, fltp, 1081 kb/s (default)
    Metadata:
      creation_time   : 2021-05-30T14:23:59.000000Z
      handler_name    : Sound Media Handler
    Stream #0:2(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, 5.1, fltp, 1074 kb/s (default)
    Metadata:
      creation_time   : 2021-05-30T14:23:59.000000Z
      handler_name    : Sound Media Handler

ffmpeg command line used:

D:\ffmpeg\bin\ffmpeg -analyzeduration 200M -probesize 200M -i snippet.mp4 -vsync vfr -map 0 -c:a copy  -c:v libx265 -preset slower -x265-params crf=21 -strict experimental snippet.mkv 

Result from ffmpeg output:

ffmpeg version 4.3.2-2021-02-27-full_build-www.gyan.dev Copyright (c) 2000-2021 the FFmpeg developers
  built with gcc 10.2.0 (Rev6, Built by MSYS2 project)
  configuration: --enable-gpl --enable-version3 --enable-static --disable-w32threads --disable-autodetect --enable-fontconfig --enable-iconv --enable-gnutls --enable-libxml2 --enable-gmp --enable-lzma --enable-libsnappy --enable-zlib --enable-libsrt --enable-libssh --enable-libzmq --enable-avisynth --enable-libbluray --enable-libcaca --enable-sdl2 --enable-libdav1d --enable-libzvbi --enable-librav1e --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxvid --enable-libaom --enable-libopenjpeg --enable-libvpx --enable-libass --enable-frei0r --enable-libfreetype --enable-libfribidi --enable-libvidstab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm --enable-cuvid --enable-ffnvcodec --enable-nvdec --enable-nvenc --enable-d3d11va --enable-dxva2 --enable-libmfx --enable-libcdio --enable-libgme --enable-libmodplug --enable-libopenmpt --enable-libopencore-amrwb --enable-libmp3lame --enable-libshine --enable-libtheora --enable-libtwolame --enable-libvo-amrwbenc --enable-libilbc --enable-libgsm --enable-libopencore-amrnb --enable-libopus --enable-libspeex --enable-libvorbis --enable-ladspa --enable-libbs2b --enable-libflite --enable-libmysofa --enable-librubberband --enable-libsoxr --enable-chromaprint
  libavutil      56. 51.100 / 56. 51.100
  libavcodec     58. 91.100 / 58. 91.100
  libavformat    58. 45.100 / 58. 45.100
  libavdevice    58. 10.100 / 58. 10.100
  libavfilter     7. 85.100 /  7. 85.100
  libswscale      5.  7.100 /  5.  7.100
  libswresample   3.  7.100 /  3.  7.100
  libpostproc    55.  7.100 / 55.  7.100
[aac @ 0000024232d47480] This stream seems to incorrectly report its last channel as LFE[6], mapping to LFE[0]
[aac @ 0000024232d50d00] This stream seems to incorrectly report its last channel as LFE[6], mapping to LFE[0]
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'snippet.mp4':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: mp42isomhvc1
    creation_time   : 2021-05-30T14:23:59.000000Z
  Duration: 00:54:23.92, start: 0.000000, bitrate: 11430 kb/s
    Stream #0:0(und): Video: hevc (Main) (hvc1 / 0x31637668), yuv420p(tv, bt709), 1920x1080 [SAR 1:1 DAR 16:9], 9269 kb/s, 23.98 fps, 24 tbr, 12288 tbn, 24 tbc (default)
    Metadata:
      creation_time   : 2021-05-30T14:23:59.000000Z
      handler_name    : Video Media Handler
      encoder         : HEVC Coding
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, 5.1, fltp, 1081 kb/s (default)
    Metadata:
      creation_time   : 2021-05-30T14:23:59.000000Z
      handler_name    : Sound Media Handler
    Stream #0:2(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, 5.1, fltp, 1074 kb/s (default)
    Metadata:
      creation_time   : 2021-05-30T14:23:59.000000Z
      handler_name    : Sound Media Handler
Stream mapping:
  Stream #0:0 -> #0:0 (hevc (native) -> hevc (libx265))
  Stream #0:1 -> #0:1 (copy)
  Stream #0:2 -> #0:2 (copy)
Press [q] to stop, [?] for help
x265 [info]: HEVC encoder version 3.4+32-b24459657
x265 [info]: build info [Windows][GCC 10.2.0][64 bit] 8bit+10bit+12bit
x265 [info]: using cpu capabilities: MMX2 SSE2Fast LZCNT SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2
x265 [info]: Main profile, Level-4 (Main tier)
x265 [info]: Thread pool created using 32 threads
x265 [info]: Slices                              : 1
x265 [info]: frame threads / pool features       : 5 / 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 / 4
x265 [info]: Keyframe min / max / scenecut / bias  : 24 / 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 / on
x265 [info]: AQ: mode / str / qg-size / cu-tree  : 2 / 1.0 / 32 / 1
x265 [info]: Rate Control / qCompress            : CRF-21.0 / 0.60
x265 [info]: tools: rect amp limit-modes rd=6 psy-rd=2.00 rdoq=2 psy-rdoq=1.00
x265 [info]: tools: rskip mode=1 limit-tu=4 signhide tmvp b-intra
x265 [info]: tools: strong-intra-smoothing deblock sao
Output #0, matroska, to 'snippet.mkv':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: mp42isomhvc1
    encoder         : Lavf58.45.100
    Stream #0:0(und): Video: hevc (libx265), yuv420p, 1920x1080 [SAR 1:1 DAR 16:9], q=-1--1, 24 fps, 1k tbn, 24 tbc (default)
    Metadata:
      creation_time   : 2021-05-30T14:23:59.000000Z
      handler_name    : Video Media Handler
      encoder         : Lavc58.91.100 libx265
    Side data:
      cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: N/A
    Stream #0:1(und): Audio: aac (LC) ([255][0][0][0] / 0x00FF), 48000 Hz, 5.1, fltp, 1081 kb/s (default)
    Metadata:
      creation_time   : 2021-05-30T14:23:59.000000Z
      handler_name    : Sound Media Handler
    Stream #0:2(und): Audio: aac (LC) ([255][0][0][0] / 0x00FF), 48000 Hz, 5.1, fltp, 1074 kb/s (default)
    Metadata:
      creation_time   : 2021-05-30T14:23:59.000000Z
      handler_name    : Sound Media Handler

As you can see, the input file has 23.98 fps, output file 24 fps. What am I missing, please?

Edit: Changing the output container to mp4 doesn't change things, unfortunately:

Output #0, mp4, to 'snippet.new.mp4':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: mp42isomhvc1
    encoder         : Lavf58.45.100
    Stream #0:0(und): Video: hevc (libx265) (hev1 / 0x31766568), yuv420p, 1920x1080 [SAR 1:1 DAR 16:9], q=-1--1, 24 fps, 12288 tbn, 24 tbc (default)
    Metadata:
      creation_time   : 2021-05-30T14:23:59.000000Z
      handler_name    : Video Media Handler
      encoder         : Lavc58.91.100 libx265
    Side data:
      cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: N/A
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, 5.1, fltp, 1081 kb/s (default)
    Metadata:
      creation_time   : 2021-05-30T14:23:59.000000Z
      handler_name    : Sound Media Handler
    Stream #0:2(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, 5.1, fltp, 1074 kb/s (default)
    Metadata:
      creation_time   : 2021-05-30T14:23:59.000000Z
      handler_name    : Sound Media Handler

Answer 1

Matroska uses a fixed timescale of 1000 (the source MP4 here uses 12288), so due to rounding, timing precision is lost and it won't be possible to represent 23.98 exactly in the target file.