I was excited to see that Mp3Tag has added support for Matroska, thanks for that! I finally had the chance to test it and I am encountering some off behavior.
I found that it would not record my changes in some files. It appeared that removing the entire tag first and then adding data back works, but that was only partially true. I honestly can't seem to get things to work the same way every time. I have seen situations where the tag panel would not update the file, but using the edit cell function in the file list would, but that was not consistent. So far my tests have been restricted to the Title, Artist, Album, and Comment fields.