I just tested this. While watching a TV episode, I brought up the OSD, went to subtitles and turned them on and selected “Set as default”.
Watched another episode and got subtitles. Rebooted and still got subtitles.
What skin are you using? Debug logs may help us see why you are not getting the subs.
To get a better understanding of the problem you are experiencing we need more information from you. The best way to get this information is for you to upload logs that demonstrate your problem. You can learn more about how to submit a useful support request here.
Depending on the used skin you have to set the settings-level to standard or higher, in summary:
enable debug logging at settings->system->logging
reboot the OSMC device
reproduce the issue
upload the log set either using the
Log Uploadermethod within the My OSMC menu in the GUI or the
sshmethod invoking command
publish the provided URL from the log set upload, here
Thanks for your understanding. We hope that we can help you get up and running again shortly.
OSMC skin screenshot: