I've release v6.7.0 of MKVToolNix. It contains tons of bug fixes all over the place, but also a couple of enhancements. The most important bug fix is probably the change to how MPLS files are handled (see below).
This is the first release for which I'm providing 64bit binaries for Windows as well. They offer the same functionality as the 32bit binaries. Versions can be distinguished in their version number output as well as mmg's »about« dialog.
Please note that the Windows installer and portable archive do not contain the word »Unicode« anymore. That's not because they don't support Unicode; they do. It's just that the last release without Unicode support was v2.something quite a lot of years ago, and there's simply no reason to point out the Unicode-awareness anymore.