Wrong packet/frame duration in some H264 video

[Laura Crema]

Summary of the bug:
Respect v. 4.3, in version 6.1 I got a wrong packet duration in some H264 video. It happens only on some files,
How to reproduce:

% ffprobe.exe -show_frames -select_streams 0 test.mp4
ffprobe version N-112960-g96f646cdc2-20231212 Copyright (c) 2007-2023 the FFmpeg developers
  built with gcc 13.2.0 (crosstool-NG 1.25.0.232_c175b21)
configuration: --prefix=/ffbuild/prefix --pkg-config-flags=--static --pkg-config=pkg-config --cross-prefix=x86_64-w64-mingw32- --arch=x86_64 --target-os=mingw32 --enable-gpl --enable-version3 --disable-debug --enable-shared --disable-static --disable-w32threads --enable-pthreads --enable-iconv --enable-libxml2 --enable-zlib --enable-libfreetype --enable-libfribidi --enable-gmp --enable-lzma --enable-fontconfig --enable-libharfbuzz --enable-libvorbis --enable-opencl --disable-libpulse --enable-libvmaf --disable-libxcb --disable-xlib --enable-amf --enable-libaom --enable-libaribb24 --enable-avisynth --enable-chromaprint --enable-libdav1d --enable-libdavs2 --disable-libfdk-aac --enable-ffnvcodec --enable-cuda-llvm --enable-frei0r --enable-libgme --enable-libkvazaar --enable-libaribcaption --enable-libass --enable-libbluray --enable-libjxl --enable-libmp3lame --enable-libopus --enable-librist --enable-libssh --enable-libtheora --enable-libvpx --enable-libwebp --enable-lv2 --enable-libvpl --enable-openal --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenh264 --enable-libopenjpeg --enable-libopenmpt --enable-librav1e --enable-librubberband --enable-schannel --enable-sdl2 --enable-libsoxr --enable-libsrt --enable-libsvtav1 --enable-libtwolame --enable-libuavs3d --disable-libdrm --enable-vaapi --enable-libvidstab --enable-vulkan --enable-libshaderc --enable-libplacebo --enable-libx264 --enable-libx265 --enable-libxavs2 --enable-libxvid --enable-libzimg --enable-libzvbi --extra-cflags=-DLIBTWOLAME_STATIC --extra-cxxflags= --extra-ldflags=-pthread --extra-ldexeflags= --extra-libs=-lgomp --extra-version=20231212
  libavutil      58. 32.100 / 58. 32.100
  libavcodec     60. 35.100 / 60. 35.100
  libavformat    60. 18.100 / 60. 18.100
  libavdevice    60.  4.100 / 60.  4.100
  libavfilter     9. 14.100 /  9. 14.100
  libswscale      7.  6.100 /  7.  6.100
  libswresample   4. 13.100 /  4. 13.100
  libpostproc    57.  4.100 / 57.  4.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'Z:\VIDEO\PAL\ONLINE\JanaTolqyn_BG_Koktem_2_03.03.2023.mp4':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: mp42mp41
    creation_time   : 2023-03-03T05:46:03.000000Z
  Duration: 00:00:10.00, start: 0.000000, bitrate: 15900 kb/s
  Stream #0:0[0x1](eng): Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, bt709), 1920x1080, 15532 kb/s, 25 fps, 25 tbr, 25k tbn (default)
    Metadata:
      creation_time   : 2023-03-03T05:46:03.000000Z
      handler_name    : ?Mainconcept Video Media Handler
      vendor_id       : [0][0][0][0]
      encoder         : AVC Coding
  Stream #0:1[0x2](eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 317 kb/s (default)
    Metadata:
      creation_time   : 2023-03-03T05:46:03.000000Z
      handler_name    : #Mainconcept MP4 Sound Media Handler
      vendor_id       : [0][0][0][0]

This is the result:

[FRAME]
media_type=video
stream_index=0
key_frame=1
pts=0
pts_time=0.000000
pkt_dts=0
pkt_dts_time=0.000000
best_effort_timestamp=0
best_effort_timestamp_time=0.000000
pkt_duration=500
pkt_duration_time=0.020000
duration=500
duration_time=0.020000
pkt_pos=61496
pkt_size=259128
width=1920
height=1080
...
[/FRAME]
[FRAME]
media_type=video
stream_index=0
key_frame=0
pts=1000
pts_time=0.040000
pkt_dts=1000
pkt_dts_time=0.040000
best_effort_timestamp=1000
best_effort_timestamp_time=0.040000
pkt_duration=500
pkt_duration_time=0.020000
duration=500
duration_time=0.020000
pkt_pos=493193
pkt_size=39086
width=1920
height=1080
...
[/FRAME]
[FRAME]
media_type=video
stream_index=0
key_frame=0
pts=2000
pts_time=0.080000
pkt_dts=2000
pkt_dts_time=0.080000
best_effort_timestamp=2000
best_effort_timestamp_time=0.080000
pkt_duration=500
pkt_duration_time=0.020000
duration=500
duration_time=0.020000
pkt_pos=532279
pkt_size=41132
width=1920
height=1080
...
[/FRAME]
....

pkt_duration and duration should be 1000 (as with version 4.3), instead I got 500

[Laura Crema]

This is an extract of a file giving the issue