A new version of mkvtoolnix has been released. Will attach ebuild... It requires new versions of libebml and libmatroska. Have a look at bug #81468 and bug #81467 Reproducible: Always Steps to Reproduce:
Created attachment 51300 [details] mkvtoolnix-1.0.2.ebuild
marking as WONTFIX, version 1.0.2 was superseded by 1.4.0. see #83521