MPEG-TS instead of MP4

[Anuskuss]

Checklist

• I'm reporting a broken site support issue
• I've verified that I'm running youtube-dl version 2020.07.28
• 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] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: [u'-o', u'video.mp4', u'-v', u'--merge-output-format', u'mp4', u'--recode-video', u'mp4', u'https://v.redd.it/16cqbkev2ci51']
[debug] Encodings: locale UTF-8, fs UTF-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2020.07.28
[debug] Python version 2.7.16 (CPython) - Linux-5.4.51-v7l+-armv7l-with-debian-10.4
[debug] exe versions: ffmpeg N-98802-g01a580f
[debug] Proxy map: {}
[RedditR] iduchl: Downloading JSON metadata
[Reddit] 16cqbkev2ci51: Downloading m3u8 information
[Reddit] 16cqbkev2ci51: Downloading MPD manifest
[debug] Default format spec: bestvideo+bestaudio/best
[debug] Invoking downloader on u'https://v.redd.it/16cqbkev2ci51/HLS_540.m3u8'
[hlsnative] Downloading m3u8 manifest
[hlsnative] Total fragments: 3
[download] Destination: video.mp4
[download] 100% of 1.98MiB in 00:01
[debug] ffmpeg command line: ffmpeg -i 'file:video.mp4'
[ffmpeg] Not converting video file video.mp4 - already is in target format mp4

Description

Most videos from v.redd.it are DASH, which means audio and video get downloaded separately and then get combined into a (working) MP4. Some videos are MPEG-TS though and youtube-dl just downloades them and leaves them alone (even though I specified -o video.mp4). Those videos play fine but they don't work as HTML5 video.
I've tried adding --merge-output-format mp4 and --recode-video mp4 but youtube-dl keeps saying

[ffmpeg] Not converting video file video.mp4 - already is in target format mp4

As a workaround I've added my own exec (youtube-dl -o tmp.mp4 --exec "ffmpeg -i {} -c copy video.mp4 && rm {}" "https://v.redd.it/16cqbkev2ci51"), which runs ffmpeg for less than a second and actually gives me a web-browser playable MP4.
Also if it help you guys to debug this: I replaced my /bin/ffmpeg with echo $* > /tmp/ffmpeg.txt and as it turns out the system ffmpeg doesn't even get called.