Well the first thing that's clear is that the ID3v2 tags don't appear correctly under mIRC because they're encoded as UTF-16-LE, which mIRC doesn't correctly handle resulting in null bytes converted to spaces.

Which would appear to be the problem: Unicode tags. I can now recreate the issue in any other software by forcing the tags to be Unicode.

Spelling mistakes, grammatical errors, and stupid comments are intentional.