Adjusting subtitle timing is available for external subtitles.
For some reason, it is not available for embedded subtitles, which would be very convenient when necessary (and that had happened to me a lot lately).
Currently, you have to find similar subtitles (or extract the embedded subtitles and put them in the right place) and then adjust timing.
I bet this is not a big development (sorry if I am wrong) and would not have any particular downside (it is just the last item in the subtitle menu anyway, and already appears for external subtitles).
Thanks in advance and have a great day!