Subtitles Displaying {\an1} Tag

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These are (horizontal?) alignment tags, which are...not technically supported by the SRT standard, but some clients/players support them. In turn, folks have taken to adding out-of-spec tags to SRT files, creating non-standard subtitle files. What's happening is you have Jellyfin serving the files properly, some of your clients going above and beyond and either stripping non-standard information or actually utilizing this information to position the subtitles OR you have what's expected -- clients reading adhering to a standard and receiving non-standard information displaying that anomaly to the end user.