Seeking with MP4 through stdout

[CharonM72]

• I've verified and I assure that I'm running youtube-dl 2018.09.18

• At least skimmed through the README, most notably the FAQ and BUGS sections

• Searched the bugtracker for similar issues including closed ones

• Checked that provided video/audio/playlist URLs (if any) are alive and playable in a browser

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• Bug report (encountered problems with youtube-dl)
• Site support request (request for adding support for a new site)
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I have a script that tries to output a crunchyroll stream to stdout. It also specifically finds the jaJP stream (which is the non-subbed stream) and use that if possible. It then pipes the stream to MPC. The command is as follows:
C:\Users\[user]\Desktop\youtube-dl.exe -o - -f best[format_id*=jaJP]/best[format_id*=enUS]/best --sub-lang enUS --ffmpeg-location "C:\Program Files (x86)\ffmpeg\bin" %path% | "C:\Program Files (x86)\MPC-HC\mpc-hc.exe" -
An example URL that works is as follows:
http://www.crunchyroll.com/darling-in-the-franxx/episode-6-darling-in-the-franxx-759585

The problem I am having is that seeking does not work at all. I know for a fact that it is possible, because SVPTube (which uses youtube-dl, but sadly is not open-source) is able to stream from crunchyroll with seeking. I have a hunch this could be a ffmpeg issue, but I could not find a solution from that end (I tried several arguments with -external-downlader-args to no avail). I have also considered that using stdout may be the issue, because SVPTube does not appear to use it, and instead has part of the video URL as the video title, which leads me to believe it may be streaming into a temporary file and playing it or something. ffmpeg is able to determine the duration of the video (it displays it when it runs) so I don't know why the media player is unable to see it.

Any assistance would be welcome.