MKV WEBM can't work with non seekable streams
MKV demux has never been designed to be used on streamed, non seekable data. It always tries to seek to find its elements. (Demuxing data also).
[00006130000bf080] mkv demux debug: MKV/Ebml Parser: m_el[mi_level] == NULL
[00006130000bf080] mkv demux error: cannot find any cluster or chapter, damaged file ?