I did some more investigating and I have found out that the tag mapping for
ID3v2 SETSUBTITLE DISCSUBTITLE
works for ID3v2.4 (Mp3Tag correctly writes it into the TSST tag) but not for ID3v2.3 which writes it into TXXX:SETSUBTITLE (this is the issue you are having).
According to Mapping – Mp3tag Documentation the ID3v2 tag name should be enough for the tag mapping. @Florian am I missing something or is this a bug with the TSST tag field? Are there any more tag fields affected like this? I tried to reproduce it with a different tag field such as Artist (TPE1) and it's mapped correctly for both ID3v2.3 and ID3v2.4 tags.
(I tested this in Mp3Tag v2.91b, I will update into 2.91d and re-test this again in the evening if needed.)