Either the bug was not fixed at all or it has reappeared. This time, the field is being truncated.
With mp3tag 2.58, the following happens:
OLD tag data:
55 46 49 44 00 00 00 0f 00 00 55 46 49 44 5f 43 52 43 33 32 00 40 f3 24 52
NEW tag data:
55 46 49 44 00 00 00 0c 00 00 55 46 49 44 5f 43 52 43 33 32 00 40
It seems, that in the advanced tag editor, the original field content is displayed truncated already.