There are
three bugs open for this already, the oldest one from the 8.0.0 release. Complain to the Arch Linux maintainers, please, it's their job to keep things up to date.
Or compile it yourself for the time being. Arch Linux is my main development platform, and the GUI works nicely there.
There's also an
AUR package called mkvtoolnix-git which builds the Qt-based GUIs.