I am trying to take the subtitle file from one version of a video and multiplex it into a second, higher-quality version which lacks subtitles. In the output file, the subtitles drift increasingly as the video goes on. I found posts that suggested the input videos may have different framerates, so I checked and the framerates are slightly different; one is 23.976014 and the other is 23.976024. It’s an absolutely miniscule difference but I guess over the length of the file it’s enough to cause a gradual desync. So I changed the fps of the higher-quality video file in the properties section in mkvtoolnix, but the output file has a framerate of 23.976216, much further from either of the sources, and definitely not what I set it to in properties.
I therefore have the following questions:
Firstly, is this tiny difference of 0.00001 fps actually the cause of the subtitles drifting? It’s such a tiny fraction I have my doubts.
If no, then how can I find out what is the cause?
If yes, then how can I make MKVToolnix actually encode the video track in the exact framerate that I set it to?