Repository: yt-dlp
Issue: Convert webvtt subtitles in mp4 into useable format
Labels: enhancement,core:post-processor

### DO NOT REMOVE OR SKIP THE ISSUE TEMPLATE

- [X] I understand that I will be **blocked** if I remove or skip any mandatory\* field

### Checklist

- [X] I'm reporting a broken site
- [X] I've verified that I'm running yt-dlp version **2022.11.11** ([update instructions](https://github.com/yt-dlp/yt-dlp#update)) or later (specify commit)
- [X] I've checked that all provided URLs are playable in a browser with the same IP and same login details
- [X] I've checked that all URLs and arguments with special characters are [properly quoted or escaped](https://github.com/yt-dlp/yt-dlp/wiki/FAQ#video-url-contains-an-ampersand--and-im-getting-some-strange-output-1-2839-or-v-is-not-recognized-as-an-internal-or-external-command)
- [X] I've searched the [bugtracker](https://github.com/yt-dlp/yt-dlp/issues?q=) for similar issues **including closed ones**. DO NOT post duplicates
- [X] I've read the [guidelines for opening an issue](https://github.com/yt-dlp/yt-dlp/blob/master/CONTRIBUTING.md#opening-an-issue)
- [X] I've read about [sharing account credentials](https://github.com/yt-dlp/yt-dlp/blob/master/CONTRIBUTING.md#are-you-willing-to-share-account-details-if-needed) and I'm willing to share it if required

### Region

USA, UK

### Provide a description that is worded well enough to be understood

I'm not sure if this problem is only specific to Discoveryplus extractor or is more of a general issue related with some specific subtitle files. The issue is that even if the subtitle is extracted and (apparently) embedded, the subtitle file is not recognized by any media player.

The format seems to be webvtt as stated inside the subtitle file (textstream_eng=1000.webvtt.fragmented.vtt@GPAC1.0.1-rev).
However, no media player o subtitle converter recognize the file as a valid subtitle file. The provided output is just an example but it happens same with any other episode that has subtitles.

If someone could check if the generated files are valid or there is some kind of issue on the extracting process we could get a better grasp of what's going on.

### Provide verbose output that clearly demonstrates the problem

- [X] Run **your** yt-dlp command with **-vU** flag added (`yt-dlp -vU <your command line>`)
- [X] Copy the WHOLE output (starting with `[debug] Command-line config`) and insert it below

### Complete Verbose Output

```shell
yt-dlp.exe -vU -f "bestvideo[tbr<=1800]+bestaudio/best[tbr<=1800]" "https://www.discoveryplus.com/video/90-day-fiance-before-the-90-days/unfinished-business" -o "./700/90 Day Fiance - Before the 90 Days/Season 5/S5 E16 - Unfinished Business.mp4" --cookies ./cookies.txt -N 10 --abort-on-unavailable-fragment --write-subs --embed-subs
[debug] Command-line config: ['-vU', '-f', 'bestvideo[tbr<=1800]+bestaudio/best[tbr<=1800]', 'https://www.discoveryplus.com/video/90-day-fiance-before-the-90-days/unfinished-business', '-o', './700/90 Day Fiance - Before the 90 Days/Season 5/S5 E16 - Unfinished Business.mp4', '--cookies', './cookies.txt', '-N', '10', '--abort-on-unavailable-fragment', '--write-subs', '--embed-subs']
[debug] Encodings: locale cp1252, fs utf-8, pref cp1252, out utf-8, error utf-8, screen utf-8
[debug] yt-dlp version 2022.11.11 [8b64402] (win_exe)
[debug] Python 3.8.10 (CPython AMD64 64bit) - Windows-10-10.0.22621-SP0 (OpenSSL 1.1.1k  25 Mar 2021)
[debug] exe versions: ffmpeg 5.1.1-essentials_build-www.gyan.dev (setts)
[debug] Optional libraries: Cryptodome-3.15.0, brotli-1.0.9, certifi-2022.09.24, mutagen-1.46.0, sqlite3-2.6.0, websockets-10.4
[debug] Proxy map: {}
[debug] Loaded 1723 extractors
[debug] Fetching release info: https://api.github.com/repos/yt-dlp/yt-dlp/releases/latest
Latest version: 2022.11.11, Current version: 2022.11.11
yt-dlp is up to date (2022.11.11)
[debug] [DiscoveryPlus] Extracting URL: https://www.discoveryplus.com/video/90-day-fiance-before-the-90-days/unfinished-business
[debug] Using fake IP 6.83.15.161 (US) as X-Forwarded-For
[DiscoveryPlus] 90-day-fiance-before-the-90-days/unfinished-business: Downloading JSON metadata
[DiscoveryPlus] 4724152: Downloading JSON metadata
[DiscoveryPlus] 90-day-fiance-before-the-90-days/unfinished-business: Downloading MPD manifest
[debug] Formats sorted by: hasvid, ie_pref, lang, quality, res, fps, hdr:12(7), vcodec:vp9.2(10), channels, acodec, filesize, fs_approx, tbr, vbr, abr, asr, proto, vext, aext, hasaud, source, id
[info] 4724152: Downloading subtitles: en
[info] 4724152: Downloading 1 format(s): dash-video=1201000+dash-audio_eng=125000
[info] Writing video subtitles to: 700\90 Day Fiance - Before the 90 Days\Season 5\S5 E16 - Unfinished Business.en.mp4
[debug] Invoking dashsegments downloader on "https://dplus-northamerica-cloudfront-gcs.prod-vod.h264.io/509958fe-2cdd-43e3-9bb4-0006704325fe/dash_clear_fmp4/x-discovery-token=Expires=1671557319&KeyName=primary&Signature=GpfLv9DIu5l76tl-YZoCeExRFb4/master.mpd"
[dashsegments] Total fragments: 89
[download] Destination: 700\90 Day Fiance - Before the 90 Days\Season 5\S5 E16 - Unfinished Business.en.mp4
[debug] File locking is not supported. Proceeding without locking
WARNING: The download speed shown is only of one thread. This is a known issue and patches are welcome
[download] 100% of   61.64KiB in 00:00:00 at 80.24KiB/s
[debug] Invoking dashsegments downloader on "https://dplus-northamerica-cloudfront-gcs.prod-vod.h264.io/509958fe-2cdd-43e3-9bb4-0006704325fe/dash_clear_fmp4/x-discovery-token=Expires=1671557319&KeyName=primary&Signature=GpfLv9DIu5l76tl-YZoCeExRFb4/master.mpd"
[dashsegments] Total fragments: 641
[download] Destination: 700\90 Day Fiance - Before the 90 Days\Season 5\S5 E16 - Unfinished Business.fdash-video=1201000.mp4
WARNING: The download speed shown is only of one thread. This is a known issue and patches are welcome
[download]  89.3% of ~ 375.03MiB at   86.67KiB/s ETA 00:07 (frag 572/641)[download] Got error: The read operation timed out. Retrying (1/10)...
[download] 100% of  365.78MiB in 00:01:13 at 4.98MiB/s
[debug] Invoking dashsegments downloader on "https://dplus-northamerica-cloudfront-gcs.prod-vod.h264.io/509958fe-2cdd-43e3-9bb4-0006704325fe/dash_clear_fmp4/x-discovery-token=Expires=1671557319&KeyName=primary&Signature=GpfLv9DIu5l76tl-YZoCeExRFb4/master.mpd"
[dashsegments] Total fragments: 642
[download] Destination: 700\90 Day Fiance - Before the 90 Days\Season 5\S5 E16 - Unfinished Business.fdash-audio_eng=125000.m4a
WARNING: The download speed shown is only of one thread. This is a known issue and patches are welcome
[download] 100% of   38.71MiB in 00:00:29 at 1.33MiB/s
[Merger] Merging formats into "700\90 Day Fiance - Before the 90 Days\Season 5\S5 E16 - Unfinished Business.mp4"
[debug] ffmpeg command line: ffmpeg -y -loglevel "repeat+info" -i "file:700\90 Day Fiance - Before the 90 Days\Season 5\S5 E16 - Unfinished Business.fdash-video=1201000.mp4" -i "file:700\90 Day Fiance - Before the 90 Days\Season 5\S5 E16 - Unfinished Business.fdash-audio_eng=125000.m4a" -c copy -map "0:v:0" -map "1:a:0" -movflags "+faststart" "file:700\90 Day Fiance - Before the 90 Days\Season 5\S5 E16 - Unfinished Business.temp.mp4"
Deleting original file 700\90 Day Fiance - Before the 90 Days\Season 5\S5 E16 - Unfinished Business.fdash-audio_eng=125000.m4a (pass -k to keep)
Deleting original file 700\90 Day Fiance - Before the 90 Days\Season 5\S5 E16 - Unfinished Business.fdash-video=1201000.mp4 (pass -k to keep)
[EmbedSubtitle] Embedding subtitles in "700\90 Day Fiance - Before the 90 Days\Season 5\S5 E16 - Unfinished Business.mp4"
[debug] ffmpeg command line: ffmpeg -y -loglevel "repeat+info" -i "file:700\90 Day Fiance - Before the 90 Days\Season 5\S5 E16 - Unfinished Business.mp4" -i "file:700\90 Day Fiance - Before the 90 Days\Season 5\S5 E16 - Unfinished Business.en.mp4" -map 0 -dn -ignore_unknown -c copy "-c:s" mov_text -map "-0:s" -map "1:0" "-metadata:s:s:0" "language=eng" -movflags "+faststart" "file:700\90 Day Fiance - Before the 90 Days\Season 5\S5 E16 - Unfinished Business.temp.mp4"
```