I hear it is an issue with the Sammy. It simply ignores the command to disable all subtitles.
There is only a labour-intensive workaround:
Strip the CC’s from the files. (Only the closed captions. Regular subtitles are OK.)
Here is a quick and dirty command to do this, using the Plex transcoder binary. (but I assume you can just use ffmpeg instead.)
INPUTFILE='/share/video/new/shows/The Lost Symbol (2021) {tmdb-120452}/Season 01/dan.browns.the.lost.symbol.s01e01.1080p.web.h264-ggez[eztv.re].mkv'
./Plex\ Transcoder -i "$INPUTFILE" -codec copy -bsf:v "filter_units=remove_types=6" "$INPUTFILE-Stripped.mkv"
Here is amore elaborate process which also converts the CCs into SRT format:
