Unable to get embedded subtitles to work

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I would recommend using the subtitle extract plugin, which will extract text-based subs to SRT format and store them next to the media files. This means you don't have to fetch anything. When subtitles are embedded, they have to be extracted ANYWAY and may be stored in cache unless you're using the aforementioned functionality. In most cases it takes a few seconds, but they are extracted in their entirety before they display at all, which can cause a large delay.

You can also check standard behavior with subtitles, as I know many clients have a setting to not display subtitles or only display the selected language regardless of defaults. You may also consider learning to use ffmpeg or, even better, mkvmerge (MKVToolNix) to strip PGS subs if you don't want them present. I do this with all my 4K media, as PGS subs will cause transcoding and tonemapping, which is a suboptimal experience.