Subtitle downloaded even when there are embedded subtitles

[dinojr]

For some files, subliminal downloads an english subtitle even though there is already an embedded subtitle in the file.

Here is the output of ffmpeg -i showing the embedded subtitle:
ffmpeg -i The\ Mandalorian\ S01E01.mp4

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'The Mandalorian S01E01.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    creation_time   : 2019-11-12T00:01:01.000000Z
    encoder         : Lavf58.34.101
  Duration: 00:38:46.21, start: 0.000000, bitrate: 3246 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, bt709), 1920x804 [SAR 1:1 DAR 160:67], 2857 kb/s, 23.98 fps, 23.98 tbr, 24k tbn, 47.95 tbc (default)
    Metadata:
      creation_time   : 2019-11-12T00:01:01.000000Z
      handler_name    : VideoHandler
    Stream #0:1(eng): Audio: ac3 (ac-3 / 0x332D6361), 48000 Hz, 5.1(side), fltp, 384 kb/s (default)
    Metadata:
      creation_time   : 2019-11-12T00:01:01.000000Z
      handler_name    : SoundHandler
    Side data:
      audio service type: main
    Stream #0:2(eng): Subtitle: mov_text (tx3g / 0x67337874), 0 kb/s (default)
    Metadata:
      creation_time   : 2019-11-12T00:01:01.000000Z
      handler_name    : SubtitleHandler
    Stream #0:3(eng): Subtitle: mov_text (tx3g / 0x67337874), 0 kb/s
    Metadata:
      creation_time   : 2019-11-12T00:01:01.000000Z
      handler_name    : SubtitleHandler

Here are some parts of the debug

subliminal --debug download -l en The\ Mandalorian\ S01E01.mp4

DEBUG:subliminal.cli:Collecting path The Mandalorian S01E01.mp4
INFO:subliminal.core:Scanning video 'The Mandalorian S01E01.mp4' in ''
DEBUG:subliminal.core:Size is 944029203
DEBUG:subliminal.core:Found subtitles {'The Mandalorian S01E01.fr.srt': <Language [fr]>}
INFO:subliminal.core:Refining video with hash
DEBUG:subliminal.refiners.hash:Computing hashes for 'The Mandalorian S01E01.mp4'
DEBUG:subliminal.refiners.hash:Computed hashes {'opensubtitles': 'fce93c51d64fe228', 'shooter': '3d8604553e1d79ff046012b67f2a5b3e;b86604b85b52b2bede4aff88c55b826e;0570296baac54cab665008eb1728dc7e;a8fcaa83e73db1c26938024a76bb3a04', 'thesubdb': '499be8e12e8160326188fb4a30403a5d'}
INFO:subliminal.core:Refining video with metadata
DEBUG:subliminal.refiners.metadata:Unsupported video extension .mp4
INFO:subliminal.core:Refining video with tvdb
INFO:subliminal.refiners.tvdb:Searching series 'The Mandalorian'
DEBUG:subliminal.refiners.tvdb:Found 2 results
DEBUG:subliminal.refiners.tvdb:Found exact match on series 'The Mandalorian'

DEBUG:subliminal.providers.tvsubtitles:Found subtitle <TVsubtitlesSubtitle '310007' [en]>
ERROR:subliminal.utils:Request timed out. Provider thesubdb
INFO:subliminal.core:Discarding provider thesubdb
INFO:subliminal.score:Computing score of <ShooterSubtitle 'https://www.shooter.cn/api/subapi.php?fetch=MTYxMDk3MTI1OHxZc1JVNHRJMDFPVjdHbm1fM3pFYVNNX3h6X3pjZHhXUXVLZ2E3R2tTc3ZtSXB6ZnVnSUpodHREaUFCOE5FOEt5SFZiSjJkU19mRFB0SjlDdmNWNFYzd0pIV2pZUExPTTZ4MjVQMzQ5aTZkVDJXX3ZqaTEtWFMtRWVmMGswLVowdFMzb2dMUT09fJT4JfWJolNjEJEGnkSsI4v28sHHUfYKcAc4NGFBiUCJ&nonce=%B9%A3p%A0%EF%BF%94V%9C%0D.%D4%FCi5%9C' [en]> for video <Episode [The Mandalorian2019 s01e01]> with {'hearing_impaired': False}
DEBUG:subliminal.score:Using scores {'hash': 809, 'series': 405, 'year': 135, 'country': 135, 'season': 45, 'episode': 45, 'release_group': 15, 'streaming_service': 15, 'source': 7, 'audio_codec': 3, 'resolution': 2, 'video_codec': 2, 'hearing_impaired': 1}
DEBUG:subliminal.score:Found matches {'hash'}
DEBUG:subliminal.score:Keeping only hash match
DEBUG:subliminal.score:Matched hearing_impaired
INFO:subliminal.score:Computed score 810 with final matches {'hash', 'hearing_impaired'}
INFO:subliminal.score:Computing score of <ShooterSubtitle 'https://www.shooter.cn/api/subapi.php?fetch=MTYxMDk3MTI1OHxTQ01UQ05GNk0xWHUxYU1zMVI4Z1dMN0gyT3BUY1UzaWw4RnMwekRuVjBSSFFuZzUyYjRHVGJ1WXJySjRCTGdzZklXSlhKUmdmdGZZa1NMM0ZsblBCNG93YXg1Sno5Y3JkQXAwbGJRUU5JX0ZSSEdtQU84SV94SGd3ZXQ5SmpfWC1tdUF2UT09fLvRgaV7BkrrytkEDX9F8v35UCXXo3MVAK-cmQTxs_LW&nonce=%D6%E7%18nkU%2C%80%2F%FE%7B%2B%E5%C6%EF%FB' [en]> for video <Episode [The Mandalorian2019 s01e01]> with {'hearing_impaired': False}
DEBUG:subliminal.score:Using scores {'hash': 809, 'series': 405, 'year': 135, 'country': 135, 'season': 45, 'episode': 45, 'release_group': 15, 'streaming_service': 15, 'source': 7, 'audio_codec': 3, 'resolution': 2, 'video_codec': 2, 'hearing_impaired': 1}
DEBUG:subliminal.score:Found matches {'hash'}
DEBUG:subliminal.score:Keeping only hash match
DEBUG:subliminal.score:Matched hearing_impaired
INFO:subliminal.score:Computed score 810 with final matches {'hash', 'hearing_impaired'}
INFO:subliminal.score:Computing score of <ShooterSubtitle 'https://www.shooter.cn/api/subapi.php?fetch=MTYxMDk3MTI1OHx6bl81UDdwU3FqdnR1RDB6X0VGVTdCb3p2c3NKalBZVU83R09Rb253UTNqNVdhSlVWM3NRdlRvQVV5a2NPVlpEV2ZzTk1EekN0dTg4bkJRbVBmanVlWTJobVhnREliN3I0ak82ZDBIT09zcXl6cTBMN0k5WkZjMlVtTWhGY3EybzJKTEtsZz09fJ0tTptnORW_gXZDK22QMr_DWudsD0yzuWFjXNPOEQab&nonce=%09%9C%FC%14%A67%22%8E%7DR%F9%9Di%CE%9F%F5' [en]> for video <Episode [The Mandalorian2019 s01e01]> with {'hearing_impaired': False}
DEBUG:subliminal.score:Using scores {'hash': 809, 'series': 405, 'year': 135, 'country': 135, 'season': 45, 'episode': 45, 'release_group': 15, 'streaming_service': 15, 'source': 7, 'audio_codec': 3, 'resolution': 2, 'video_codec': 2, 'hearing_impaired': 1}
DEBUG:subliminal.score:Found matches {'hash'}
DEBUG:subliminal.score:Keeping only hash match
DEBUG:subliminal.score:Matched hearing_impaired
INFO:subliminal.score:Computed score 810 with final matches {'hash', 'hearing_impaired'}
INFO:subliminal.score:Computing score of <TVsubtitlesSubtitle '310007' [en]> for video <Episode [The Mandalorian2019 s01e01]> with {'hearing_impaired': False}
DEBUG:subliminal.score:Using scores {'hash': 809, 'series': 405, 'year': 135, 'country': 135, 'season': 45, 'episode': 45, 'release_group': 15, 'streaming_service': 15, 'source': 7, 'audio_codec': 3, 'resolution': 2, 'video_codec': 2, 'hearing_impaired': 1}
DEBUG:subliminal.score:Found matches {'season', 'series', 'country', 'episode', 'year'}
DEBUG:subliminal.score:Matched hearing_impaired
INFO:subliminal.score:Computed score 766 with final matches {'season', 'series', 'hearing_impaired', 'country', 'episode', 'year'}
INFO:subliminal.core:Downloading subtitle <ShooterSubtitle 'https://www.shooter.cn/api/subapi.php?fetch=MTYxMDk3MTI1OHxZc1JVNHRJMDFPVjdHbm1fM3pFYVNNX3h6X3pjZHhXUXVLZ2E3R2tTc3ZtSXB6ZnVnSUpodHREaUFCOE5FOEt5SFZiSjJkU19mRFB0SjlDdmNWNFYzd0pIV2pZUExPTTZ4MjVQMzQ5aTZkVDJXX3ZqaTEtWFMtRWVmMGswLVowdFMzb2dMUT09fJT4JfWJolNjEJEGnkSsI4v28sHHUfYKcAc4NGFBiUCJ&nonce=%B9%A3p%A0%EF%BF%94V%9C%0D.%D4%FCi5%9C' [en]>
INFO:subliminal.providers.shooter:Downloading subtitle <ShooterSubtitle 'https://www.shooter.cn/api/subapi.php?fetch=MTYxMDk3MTI1OHxZc1JVNHRJMDFPVjdHbm1fM3pFYVNNX3h6X3pjZHhXUXVLZ2E3R2tTc3ZtSXB6ZnVnSUpodHREaUFCOE5FOEt5SFZiSjJkU19mRFB0SjlDdmNWNFYzd0pIV2pZUExPTTZ4MjVQMzQ5aTZkVDJXX3ZqaTEtWFMtRWVmMGswLVowdFMzb2dMUT09fJT4JfWJolNjEJEGnkSsI4v28sHHUfYKcAc4NGFBiUCJ&nonce=%B9%A3p%A0%EF%BF%94V%9C%0D.%D4%FCi5%9C' [en]>
INFO:subliminal.subtitle:Guessing encoding for language en
DEBUG:subliminal.subtitle:Trying encodings ['utf-8', 'latin-1']
INFO:subliminal.subtitle:Guessed encoding latin-1
INFO:subliminal.subtitle:Guessing encoding for language en
DEBUG:subliminal.subtitle:Trying encodings ['utf-8', 'latin-1']
INFO:subliminal.subtitle:Guessed encoding latin-1
DEBUG:subliminal.core:All languages downloaded
Downloading subtitles  [####################################]  100%               

[dinojr]

It turns the problem comes from the tx3g subtitle format embedded in the mp4 video. The same video transcoded to a mkv container, with the subtitle embedded in ass format with handbrake doesn't have this problem.