temporary fix using Subler (suitable for .mks files or .mkv files containing subtitle tracks)
- Download the latest Subler
- Open and go to Preferences > OCR > Download relevant languages
- Go to Preferences > Advanced, and make sure VobSub to Tx3g is enabled
- Select Open and open your MKS or MKV file
- Deselect all tracks except the subtitle track
- Choose File > Save, leave extension as .m4v and confirm
- It will now OCR the subtitles and save a .m4v file
- When it's done, highlight the subtitle track and choose File > EXPORT. This will save as .srt