Srt subtitles are not displayed correctly

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This is likely due to the subtitle extraction process running in the background when you start the media. This can take some time, regardless of the power of your machine. If you have "extract subtitles on the fly" enabled, when you start the media, Jellyfin starts playing the media while extracting the subs in the background.

If, when you press a button to make the player controls appear, you see "Fetching additional information" and a spinning wheel, it's busy extracting the subs. Just be patient.