ffprobe the working MP4 (-vf colormatrix=bt601:bt709 -pix_fmt yuv420p)

probe_mp4_420_bt709

>ffprobe -show_format sample_420_bt709.mp4
ffprobe version N-81045-g450cf40 Copyright (c) 2007-2016 the FFmpeg developers
  built with gcc 5.4.0 (GCC)
  configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-dx
va2 --enable-libmfx --enable-nvenc --enable-avisynth --enable-bzlib --enable-lib
ebur128 --enable-fontconfig --enable-frei0r --enable-gnutls --enable-iconv --ena
ble-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libfree
type --enable-libgme --enable-libgsm --enable-libilbc --enable-libmodplug --enab
le-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-lib
openjpeg --enable-libopus --enable-librtmp --enable-libschroedinger --enable-lib
snappy --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame
 --enable-libvidstab --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx
--enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-
libxavs --enable-libxvid --enable-libzimg --enable-lzma --enable-decklink --enab
le-zlib
  libavutil      55. 28.100 / 55. 28.100
  libavcodec     57. 50.100 / 57. 50.100
  libavformat    57. 42.100 / 57. 42.100
  libavdevice    57.  0.102 / 57.  0.102
  libavfilter     6. 47.100 /  6. 47.100
  libswscale      4.  1.100 /  4.  1.100
  libswresample   2.  1.100 /  2.  1.100
  libpostproc    54.  0.100 / 54.  0.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'sample_420_bt709.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf57.42.100
  Duration: 00:00:02.60, start: 0.000000, bitrate: 1196 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1920x1080
 [SAR 1:1 DAR 16:9], 1191 kb/s, 25 fps, 25 tbr, 12800 tbn, 50 tbc (default)
    Metadata:
      handler_name    : VideoHandler
[FORMAT]
filename=sample_420_bt709.mp4
nb_streams=1
nb_programs=0
format_name=mov,mp4,m4a,3gp,3g2,mj2
format_long_name=QuickTime / MOV
start_time=0.000000
duration=2.600000
size=388813
bit_rate=1196347
probe_score=100
TAG:major_brand=isom
TAG:minor_version=512
TAG:compatible_brands=isomiso2avc1mp41
TAG:encoder=Lavf57.42.100
[/FORMAT]