Few are the changes available in the newest release for MKVToolNix, version 5.7, but the entries in the change log also show that a new feature has been introduced in the package. Repairs are also on the menu, touching on MMG (mkvmerge and mkvmerge GUI).
These refer to a bug that caused MMG to print false warning about a chapter UID not being unique. The second fix available in the release enables support for 64-bit UID values (chapter UIDs, edition UIDs, etc.) for all the tools included in the bundle.
As for the new feature, it is assigned to MKVmerge component and it allows the module to finish muxing if "splitting by parts" is active and the last split part has a finite end point.
MKVToolNix is available for download on this page