Please give us the option of disabling on the server side the option to burn subtitles. This is very annoying, i never had any trouble with that but lately even viewer without subtitle often get them activated and 1 stream takes 100% cpu because of that.
Not sure what player you are using or what type fo subtitles. but some players cannot play all types of subtitles and need to burn them in if you want to see subtitles at all. Some players require everything to be transcoded if any of the file like audio need to be transcoded depending on multiple factors.
most players will have an option in player settings to only burn image based subs like PGS if it can play text based.
Can you get the server logs from after one of them starts playback with subtitles set to none, and the xml info for whichever file they were playing. The logs should indicate why it started transcoding an hopefully why it thought subtitles were enabled