I think it is not a bug in MediaPortal but maybe in LAVF and/or a driver issue. But @tourettes knows a lot more about this stuff.
I am fine leaving this as a LAV issue and not a MP issue. But if there is any testing or logs that need to be performed, let me know, even if as a side bar.
Not much can be tested on MP side (at least on log level) - I would need to have some HW that I can test / reproduce the issue.
Could you do the following test?
1) Open GraphEdit
2) Load LAV source filter (select Blu-rays .bmdv)
3) Add LAV audio decoder to graph
4) Add LAV video decoder to graph
5) Add DS audio renderer to graph
6) Add EVR video renderer to graph
7) Connect all pins
8) Play
Does that have working bitstreaming?