The VideoLAN Forums

Subtitle Option, make selected sub stick

this affects the MKV subtitles,

most MKV have more than one Subtitle,

i hope there will be an option to make the selected Subtitle or
Selected order of that sub stick

MKV subtitles don't exist.
Check for the subtitles format being used inside the MKV container with MediaInfo:
https://mediaarea.net/en/MediaInfo/Download