When last stop is 0 the demuxer will skip the seeking part, the pipeline will be flushed and demuxing will continue with the next frame in the file which can be a delta-unit:

gst_matroska_demux_handle_seek_event () {
...
  if (!update) {
    /* only have to update some segment,
     * but also still have to honour flush and so on */
    GST_DEBUG_OBJECT (demux, "... no update");
    /* bad goto, bad ... */
    goto next;
  }
...
}