a few days ago, i put every file i had in a playlist and selected them all for mass tagging, all i told masstagger to do was remove comments field and composer field.
it took about 15 mins to get through 3000 files, when i came to use my files after this, i found that fb2k had removed all tags from all files, not just the ones i had asked it to do.

using 0.8.3 full.

any ideas?

Can you reproduce this behaviour?

Otherwise it's likely that you've accidentally commanded it to remove the tags.