Hi, I don't remember if I was able to edit them in the past but I know that I can edit them on Windows. Am hoping that I can have the ability to edit MKV files on the Mac version that I purchased. I'm on version 1.8.22 for MacOS, latest build.
I've just released Mp3tag for Mac v1.9 with support to view and edit metadata of Matroska audio (MKA), Matroska video (MKV), and WebM files. Support for chapters in Matroska is on my internal wish list.