I'm curious as to what the correct usage of this Tag frame is, especially within the context of MP3Tag. I know that according to the ID3v2.3 specifications, that it's supposed to be pairs of involvement types and the person involved, with both strings being terminated. So it should appear something like "Producer[null]John Smith[null]Engineer[null]David Johson[null]" in the tag itself. However, as with many other frames, the way they're written and the way they're entered/displayed are very different.
A lot of programs use the format of "Involvement:Person;Involvement:Person" when it comes to entering and displaying this information. So entering "Producer:YoungFyre;Producer:Karbon" in the Involved People frame in MP3Tag, it correctly writes to the tag as "Producer[null]YoungFyre[null]Producer[null]Karbon" (which causes a compatibility issue with another program, but that's really due to their "improper" implementation of the field). However, looking at this frame in MP3Tag after saving it shows "Producer:YoungFyre;Producer:Karbon;:" suggesting that what I believed to be the proper usage... isn't. Why is there a ";:" at the end?