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[Broken] Wrong timestamps on WebVTT parsed subtitles

See original GitHub issue

Checklist

I’m reporting a broken site support issue
I’ve verified that I’m running yt-dlp version 2021.07.07
I’ve checked that all provided URLs are alive and playable in a browser
I’ve checked that all URLs and arguments with special characters are properly quoted or escaped
I’ve searched the bugtracker for similar bug reports including closed ones
I’ve read bugs section in FAQ

Verbose log

[debug] Command-line config: ['--write-subs', '--skip-download', '-v', 'https://devstreaming-cdn.apple.com/videos/streaming/examples/img_bipbop_adv_example_ts/master.m3u8']
[debug] Encodings: locale cp1252, fs utf-8, out utf-8, pref cp1252
[debug] yt-dlp version 2021.07.07 (exe)
[debug] Plugin Extractors: ['SamplePlugin']
[debug] Python version 3.8.10 (CPython 64bit) - Windows-10-10.0.19043-SP0
[debug] exe versions: ffmpeg 2021-06-09-git-e01bf559df-full_build-www.gyan.dev, ffprobe 2021-06-09-git-e01bf559df-full_build-www.gyan.dev
[debug] Proxy map: {}
[debug] [generic] Extracting URL: https://devstreaming-cdn.apple.com/videos/streaming/examples/img_bipbop_adv_example_ts/master.m3u8
[generic] master: Requesting header
[generic] master: Downloading m3u8 information
[debug] Formats sorted by: hasvid, ie_pref, lang, quality, res, fps, vcodec:vp9.2(10), acodec, filesize, fs_approx, tbr, vbr, abr, asr, proto, vext, aext, hasaud, source, id
[debug] Downloading subtitles: en
[debug] Default format spec: bestvideo*+bestaudio/best
[info] master: Downloading 1 format(s): 8366+aud3-English
[info] Writing video subtitles to: master [master].en.vtt
[debug] Invoking downloader on "https://devstreaming-cdn.apple.com/videos/streaming/examples/img_bipbop_adv_example_ts/s1/en/prog_index.m3u8"
[hlsnative] Downloading m3u8 manifest
[hlsnative] Total fragments: 100
[download] Destination: master [master].en.vtt
[download] 100% of 29.83KiB in 02:02

Description

Parsed WebVTT subtitles have the wrong timestamps. Expected (not parsed):

WEBVTT
X-TIMESTAMP-MAP=MPEGTS:900000,LOCAL:00:00:00.000

1
00:00:00.008 --> 00:00:00.992
Subtitles: Bip!

2
00:00:01.000 --> 00:00:01.992
Subtitles: Bop!

3
00:00:02.000 --> 00:00:02.992
Subtitles: Bip!

4
00:00:03.000 --> 00:00:03.992
Subtitles: Bop!

...

Got:

WEBVTT
X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000

1
08:00:00.000 --> 32:00:16.000
Subtitles: Bip!

2
00:00:00.001 --> 32:00:16.001
Subtitles: Bop!

3
00:00:00.002 --> 32:00:16.002
Subtitles: Bip!

4
00:00:00.003 --> 32:00:16.003
Subtitles: Bop!

...

Also observed in: https://www.vidio.com/watch/2234540-stand-by-me-doraemon-2 (geolocked and paywalled, verified by me) https://roosterteeth.com/watch/gen-lock-season-1-1 (semi-geolocked, verified by XMarksTheSpot (discord), @nao20010128nao, and me)