Gibberish Closed Captioning subtitle

[macdavis]

Summary of the bug:
CC Subtitle is garbled. The subtitle position is also inappropriate.

How to reproduce:
Play "wwdc_2017_1080_cc.m4v" with MPV (download from ​http://podcasts.apple.com/apple_keynotes_1080p/2017/wwdc_2017_1080_cc.m4v)

The issue was initially reported to MPV develepers (link: ​https://github.com/mpv-player/mpv/issues/4493). One of the developers suggests submit it here as "FFmpeg also fails to decode anything useful from it."

[macdavis]

ffmpeg -i http://podcasts.apple.com/apple_keynotes_1080p/2017/wwdc_2017_1080_cc.m4v -vn -an out.srt
ffmpeg version 3.3.2 Copyright (c) 2000-2017 the FFmpeg developers
  built with Apple LLVM version 8.1.0 (clang-802.0.42)
  configuration: --prefix=/usr/local --extra-cflags='-march=native -mtune=intel -m64 -O3 -pipe -fomit-frame-pointer -ftree-vectorize' --disable-runtime-cpudetect --disable-debug --disable-opencl --disable-avdevice --disable-ffplay --disable-ffprobe --disable-postproc --disable-ffserver --disable-devices --disable-avresample --disable-indev=qtkit --disable-doc --enable-pthreads --enable-version3 --enable-hardcoded-tables --enable-gpl --enable-nonfree
  libavutil      55. 58.100 / 55. 58.100
  libavcodec     57. 89.100 / 57. 89.100
  libavformat    57. 71.100 / 57. 71.100
  libavfilter     6. 82.100 /  6. 82.100
  libswscale      4.  6.100 /  4.  6.100
  libswresample   2.  7.100 /  2.  7.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'http://podcasts.apple.com/apple_keynotes_1080p/2017/wwdc_2017_1080_cc.m4v':
  Metadata:
    major_brand     : qt  
    minor_version   : 537199360
    compatible_brands: qt  
    creation_time   : 2017-06-06T23:34:44.000000Z
    com.apple.quicktime.player.movie.audio.gain: 1.000000
    com.apple.quicktime.player.movie.audio.treble: 0.000000
    com.apple.quicktime.player.movie.audio.bass: 0.000000
    com.apple.quicktime.player.movie.audio.balance: 0.000000
    com.apple.quicktime.player.movie.audio.pitchshift: 0.000000
    com.apple.quicktime.player.movie.audio.mute: 
    com.apple.quicktime.player.movie.visual.brightness: 0.000000
    com.apple.quicktime.player.movie.visual.color: 1.000000
    com.apple.quicktime.player.movie.visual.tint: 0.000000
    com.apple.quicktime.player.movie.visual.contrast: 1.000000
    com.apple.quicktime.player.version: 7.6.6 (7.6.6)
    com.apple.quicktime.version: 7.7.3 (2978.7) 0x7738000 (Mac OS X, 10.12.5, 16F73)
  Duration: 02:19:04.60, start: -0.012000, bitrate: 8099 kb/s
    Stream #0:0: Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1920x800 [SAR 1:1 DAR 12:5], 8000 kb/s, 29.97 fps, 29.97 tbr, 30k tbn, 59.94 tbc (default)
    Metadata:
      creation_time   : 2017-06-06T23:34:44.000000Z
      handler_name    : Apple Alias Data Handler
      encoder         : Elemental H.264
    Stream #0:1: Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 93 kb/s (default)
    Metadata:
      creation_time   : 2017-06-06T23:34:44.000000Z
      handler_name    : Apple Alias Data Handler
    Stream #0:2(eng): Subtitle: eia_608 (c608 / 0x38303663), 720x480, 0 kb/s, SAR 174763:109227 DAR 174763:72818 (default)
    Metadata:
      rotate          : 0
      creation_time   : 2017-06-06T23:34:44.000000Z
      handler_name    : Apple Alias Data Handler
Output #0, srt, to 'out.srt':
  Metadata:
    major_brand     : qt  
    minor_version   : 537199360
    compatible_brands: qt  
    com.apple.quicktime.version: 7.7.3 (2978.7) 0x7738000 (Mac OS X, 10.12.5, 16F73)
    com.apple.quicktime.player.movie.audio.gain: 1.000000
    com.apple.quicktime.player.movie.audio.treble: 0.000000
    com.apple.quicktime.player.movie.audio.bass: 0.000000
    com.apple.quicktime.player.movie.audio.balance: 0.000000
    com.apple.quicktime.player.movie.audio.pitchshift: 0.000000
    com.apple.quicktime.player.movie.audio.mute: 
    com.apple.quicktime.player.movie.visual.brightness: 0.000000
    com.apple.quicktime.player.movie.visual.color: 1.000000
    com.apple.quicktime.player.movie.visual.tint: 0.000000
    com.apple.quicktime.player.movie.visual.contrast: 1.000000
    com.apple.quicktime.player.version: 7.6.6 (7.6.6)
    encoder         : Lavf57.71.100
    Stream #0:0(eng): Subtitle: subrip (srt), 720x480 (default)
    Metadata:
      encoder         : Lavc57.89.100 srt
      creation_time   : 2017-06-06T23:34:44.000000Z
      handler_name    : Apple Alias Data Handler
Stream mapping:
  Stream #0:2 -> #0:0 (eia_608 (cc_dec) -> subrip (srt))
Press [q] to stop, [?] for help
[Closed caption Decoder @ 0x7fa9fb009200] Data Ignored since exceeding screen width
    Last message repeated 7 times
[Closed caption Decoder @ 0x7fa9fb009200] Data Ignored since exceeding screen width
    Last message repeated 43 times
[Closed caption Decoder @ 0x7fa9fb009200] Data Ignored since exceeding screen width
    Last message repeated 27 times
[Closed caption Decoder @ 0x7fa9fb009200] Data Ignored since exceeding screen width
    Last message repeated 33 times
[Closed caption Decoder @ 0x7fa9fb009200] Data Ignored since exceeding screen width
    Last message repeated 19 times
... #(repeated messages and I have to terminate it)

size=       0kB time=00:00:00.00 bitrate=N/A speed=   0x    
video:0kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown
Output file is empty, nothing was encoded (check -ss / -t / -frames parameters if used)
Exiting normally, received signal 2