Hi Ralph,
So you have Vobsub within MKV. The "sub" file contains the bitmaps (graphical subs), the "id
x" file contains the timings. I will check this constellation on my side later when I'm home.
Anyhow, some general remarks:
1.) vobsubs are ugly (it's the DVD subtitle format with it's limited resolution).
2.) If you find srt subtitles for the same movie, I would just replace them.
3.) You can create srt from vobsub via OCR; there are different tools doing this, for example SubtitleEdit (3.5.2), SubRip (1.56.1), Subtitle Extractor (1032d).
4.) With the latter tools, you can also check if the timings in the idx are the issue or if they are correct and HiMedia has an issue...
But as said, I will double check this on my own, as I find a stable subitle support / implementation one of the most important features at all (and not playing italic srt subtitles for example let's me massively rethink my latest Q5Pro buy and consider to look around once more

).
Hope it helps
Detlev