CodecPrivate compression in Matroska is not supported
In Matroska the compression can apply to the Frames, the CodecPrivate or both. When the CodecPrivate is compressed VLC doesn't seem to decompress it and use the compressed data as is. I attach files that exhibit this behaviour.
You can create the same files using 'mkclean --optimize'. You can uncompress these files using 'mkclean --no-optimize'.