A minor software update for Apple TV fixes an issue with non-functional and delayed subtitles across various apps such as HBO. The bug was introduced with tvOS 12.
tvOS 12.0.1 for Apple TV
Since the release of tvOS 12, users have been complaining about a bug that has caused subtitles to be delayed or disappear entirely. It has affected a range of video apps such as HBO as well as Viaplay and C More in the Nordic. However, not all apps were affected. Netflix steered clear of trouble.
Apple has not released a change log for the minor tvOS 12.0.1 software update but it appears that the issue has been fixed, suggesting that the fault was Apple’s own. Users have contacted FlatpanelsHD to let us know that subtitles now work as intended. Users on Apple’s discussion board are reporting the same.

With this bug out of the way, it should now be safe to install tvOS 12 on your Apple TV4 or Apple TV 4K. FlatpanelsHD is not aware of any other major bugs in the latest version of Apple’s TV operating system.
tvOS 12 was released on September 17, with the headlining feature being Dolby Atmos support, and tvOS 12.0.1 followed on September 24. Apple has also issued the first beta of tvOS 12.1 but we have yet to discover any new features.