In Resolve 19.1.4, render caching completely ignores Subtitles in nested sequences. So if you have render caching enabled, and you happen to have a nested sequence that contains Subtitles that you want to have "burned in" at all times, even when the sequence is used as a nest, disappear completely when the nest is cached.
I would expect render caching to do whatever it needs to do in order to cache frames without affecting the appearance of the sequence itself.
Here's a short demo of the "bug" in action (if it's indeed a bug, and not intended behavior):