A new version of media-video/mkvtoolnix is out, please bump.
This version requires qt5 when the qt USE flag is enabled. Seems like upstream completely removed qt4 support without even noticing this in their release notes *sigh*
6.9.0 added to the tree masked for now waiting for either qt5 to be in the tree or me to disable/remove qt support from it temporarily.
6.9.1 has been released.