Hi @Sibby,
A warm welcome to the Roku Community!
Thanks for letting us know about this behavior you have encountered with YouTube subtitles not functioning properly. We would be more than happy to assist. We recommend performing the steps below to resolve the issue.
1. Remove the channel by highlighting it and pressing the star (*) button.
2. Restart the Roku device from Settings > System > System Restart.
3. Install the channel again.
One additional suggestion we would like to recommend is to perform a system update. You can check for a manual system update by navigating to Home > Settings > System > System Update > Check Now. Kindly check and see if you notice any difference.
Keep us posted on how it works.
All the best,
Kash
Takashi O.
Roku Community Moderator