[ViX_Misc] Playing MKV's doesn't pick up PGS sub titles

[twol]

This I think is a generic problem, as the issue arose from a Dreambox user and when I tried on my VIX image I had the same problem.
When playing a MKV bluray video with multiple language audio and subtitle tracks, although the player shows all the audio tracks (and will play any of them), it only shows one "generic" PGS track (marked as selected) and does not give any further language choice and does not play any subtitles.

Any ideas?