yup I know that this is a superlow priority bug, but since it works for all unmapped fields, I found it worth mentioning.
To reproduce this, first configure a field-mapping. For example, MIXARTIST to REMIXED BY for ID3v2. Next, load up an audiofile using the tagtype you've mapped (mp3 in this case) and write test\\test in the tag panel field of REMIXED BY (or create two values for REMIXED BY at the extended tags window). You'll see that only the first, but not the second value will be written to the tag.
Entering two different values works normally.