In the matroskamux plugin, upon receiving an EOS it closes out the file, writing various data, including a TAGS section.  The code to write this TAGS section will write the header for the section to the file even if there are no valid matroska tags in the tag list.  

This is because in matroska-mux.c, in the gst_matroska_mux_finish() function, it checks to make sure tags != NULL, then writes the headers, then iterates through the tag_list and attempts to write each tags to file.  However, the code to write the tags - gst_matroska_mux_write_simple_tag() - will only write 15 specific tags to this section.  If none of those tags exist, the result is an empty TAGS section with the headers indicating that there should be data available.  This causes errors to be reported in several matroska validators.

A workaround is to use the following:

GstTagSetter *tagsetter = GST_TAG_SETTER(mux);
gst_tag_setter_reset_tags(tagsetter);