I've unselected ID3v1 and tried again - that works. However it also creates a bunch of files with the extension mp3.xxxxxx and no ID info, which are copies of the original mp3 files. Looks like a bug to me.
Not a bug. You have used the correct procedure, Mixvibes generates the .XXXXXX files in place of songs with incompatible tag types, if you rename the .XXXXXX files the songs will work with the saved information... the reason for this is Mixvibes can’t save the same file name twice.
Example,
Your – song.mp3.XXXXXX
Change this to,
Your – song.mp3
Removing the .XXXXXX will return it to a normal mp3, you will have to remove the original files first.
Also you can only output one tag type at a time, always best to go for ID3v2 because this tag holds more information that the original tag types such as ID3v1 or ID3v1.1.