FFMPEG slideshow skips a picture when showing on VLC (and is OK on Quicktime)

Question

I created a slideshow following the directions on the concat demuxer. The input file is:

file 'sentence1.jpg
duration 5
file 'sentence2.jpg
duration 5
file 'sentence3.jpg
duration 5
file '30.jpg
duration 5
file '40.jpg
duration 5
file '50.jpg
duration 5
file '60.jpg
duration 5
file '70.jpg
duration 5
file '30.jpg
duration 5
file '40.jpg
duration 5
file '50.jpg
duration 5
file '50.jpg

The result looks OK on QuickTime with 5 seconds for each of the first three pictures. For the same file, VLC shows 10 seconds for the first picture, a flash of the second, and the third for 5 seconds.

I could display the video on QuickTime or use another non-linear editor to compile the slideshow. But I'm curious about this quirk.

Has anyone faced this problem? How can I diagnose it?

The version of VLC is Version 3.0.8 Vetinari (Intel 64bit). The log of the command is:

$ ffmpeg -f concat -i slideshow2.txt  -vsync vfr -pix_fmt yuv420p output2.mp4
ffmpeg version 4.2.2 Copyright (c) 2000-2019 the FFmpeg developers
  built with Apple clang version 11.0.0 (clang-1100.0.33.16)
  configuration: --prefix=/usr/local/Cellar/ffmpeg/4.2.2 --enable-shared --enable-pthreads --enable-version3 --enable-avresample --cc=clang --host-cflags='-I/Library/Java/JavaVirtualMachines/adoptopenjdk-13.0.1.jdk/Contents/Home/include -I/Library/Java/JavaVirtualMachines/adoptopenjdk-13.0.1.jdk/Contents/Home/include/darwin -fno-stack-check' --host-ldflags= --enable-ffplay --enable-gnutls --enable-gpl --enable-libaom --enable-libbluray --enable-libmp3lame --enable-libopus --enable-librubberband --enable-libsnappy --enable-libtesseract --enable-libtheora --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libx265 --enable-libxvid --enable-lzma --enable-libfontconfig --enable-libfreetype --enable-frei0r --enable-libass --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-librtmp --enable-libspeex --enable-libsoxr --enable-videotoolbox --disable-libjack --disable-indev=jack
  libavutil      56. 31.100 / 56. 31.100
  libavcodec     58. 54.100 / 58. 54.100
  libavformat    58. 29.100 / 58. 29.100
  libavdevice    58.  8.100 / 58.  8.100
  libavfilter     7. 57.100 /  7. 57.100
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  5.100 /  5.  5.100
  libswresample   3.  5.100 /  3.  5.100
  libpostproc    55.  5.100 / 55.  5.100
Input #0, concat, from 'slideshow2.txt':
  Duration: N/A, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: mjpeg (Baseline), yuvj420p(pc, bt470bg/unknown/unknown), 4032x3024, 25 tbr, 25 tbn, 25 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (mjpeg (native) -> h264 (libx264))
Press [q] to stop, [?] for help
[swscaler @ 0x111f69000] deprecated pixel format used, make sure you did set range correctly
[libx264 @ 0x7fc2fd818000] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2
[libx264 @ 0x7fc2fd818000] profile High, level 6.0
[libx264 @ 0x7fc2fd818000] 264 - core 155 r2917 0a84d98 - H.264/MPEG-4 AVC codec - Copyleft 2003-2018 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=6 lookahead_threads=1 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=25 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, mp4, to 'output2.mp4':
  Metadata:
    encoder         : Lavf58.29.100
    Stream #0:0: Video: h264 (libx264) (avc1 / 0x31637661), yuv420p, 4032x3024, q=-1--1, 25 fps, 12800 tbn, 25 tbc
    Metadata:
      encoder         : Lavc58.54.100 libx264
    Side data:
      cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: -1
[swscaler @ 0x111f69000] deprecated pixel format used, make sure you did set range correctly
frame=    5 fps=0.0 q=0.0 size=       0kB time=00:00:00.00 bitrate=N/A speed=   frame=   10 fps=8.6 q=0.0 size=       0kB time=00:00:00.00 bitrate=N/A speed=   frame=   12 fps=1.0 q=-1.0 Lsize=   16739kB time=00:00:45.00 bitrate=3047.2kbits/s speed=3.73x    
video:16738kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.005508%
[libx264 @ 0x7fc2fd818000] frame I:2     Avg QP:12.59  size:1762396
[libx264 @ 0x7fc2fd818000] frame P:10    Avg QP:15.19  size:1361412
[libx264 @ 0x7fc2fd818000] mb I  I16..4:  0.2% 96.8%  2.9%
[libx264 @ 0x7fc2fd818000] mb P  I16..4:  0.1% 85.8%  4.0%  P16..4:  0.1%  0.0%  0.0%  0.0%  0.0%    skip:10.0%
[libx264 @ 0x7fc2fd818000] 8x8 transform intra:95.7% inter:85.7%
[libx264 @ 0x7fc2fd818000] coded y,uvDC,uvAC intra: 99.2% 57.5% 27.3% inter: 0.6% 0.8% 0.0%
[libx264 @ 0x7fc2fd818000] i16 v,h,dc,p: 12% 21% 28% 38%
[libx264 @ 0x7fc2fd818000] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 10% 35% 19%  4%  5%  4%  8%  5% 11%
[libx264 @ 0x7fc2fd818000] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 17% 22%  8%  7%  9%  8% 10%  8% 10%
[libx264 @ 0x7fc2fd818000] i8c dc,h,v,p: 46% 28% 22%  4%
[libx264 @ 0x7fc2fd818000] Weighted P-Frames: Y:0.0% UV:0.0%
[libx264 @ 0x7fc2fd818000] ref P L0: 64.1% 12.7% 20.0%  3.3%
[libx264 @ 0x7fc2fd818000] kb/s:2285.19