It seems that google (youtube) is giving a complete different website code structure for some account (few people at the moment), which is screwing LR extension.
Luckily, we do have an account that has the same problem, so we are working on fixing this.
@Gini_Lin , @Petey , can you try to logout from YT of your youtube/google account, and try with no account (or another one) if the subtitles are coming back? If it’s the same problem as us, it should be working.
We will let you informed, sorry for this :S