Despite its age, Matroska is still evolving, adding new features or making the specifications more precise.
It seems our recent changes on the StereoMode element of the 3D specifications broke compatibility with some systems that were using the 'experimental' system that was there for ages. We were not aware Matroska was already used for 3D movies in many forms. As a result it's becoming difficult for these systems to evolve to the new system and keep compatibility with their old files.
That's why we are calling everyone who is working on Matroska is they are OK with moving the values 1,2 and 3 to different values. On our side that means issuing a new version of mkvtoolnix changing these values. We are not aware of any (other) player that is already making use of these values. But if you do, please contact us ASAP so we can coordinate the fix.
In other news we added a LYRICS entity to the tags system, that could be a useful addition to the subtitles in music files.
We already updated the ordering guidelines to say that double Meta Seek are deprecated and that Meta Seek should contain some padding to leave room for missing elements at the time of the original muxing (like tags, attachments and others).