It seems like this code has been abandoned (5 years since the last commit), but the patch duck linked to works. If any maintainers are still around, it would be great to get it committed.
Even with --ParseSpeed=1, captions are not detected. I've sent you an email regarding...
Captions not detected in some LXF files
I was also able to extract the attached caption data using Telestream Vantage.
I get good 608 captions out when I play it back on an Imagine Nexio NX3601 running...
Also note - the first caption is around 3s from the start of the file.
608 captions not detected (mov container)
LXF container support