On some bardsongs there is a bonus damage applied to the weapon and not the character. These bonuses only work when the bard sings for himself only, as far as I have found. I assume it is a bug, and I can list the songs (that I know) if needed.
I have had the same problem on both Alvin Anderson and Aorli Brakenshield and can name at least three songs with this issue. On a related note, one song appears to have lost its bonus AB.
Still a problem.
People have reported this before. It works only if you're the only character affected by the song :(.