ok, mono -> stereo converting is not a bug, but stereo -> mono is a bug, and has been fixed.
re: mono -> stereo
we decided to not allow this, because it makes no sense - there is no extra information included into your mp3 files, all it does is take the existing mono channel and duplicates it. What you end up with is the one channel duplicated, the mp3 file size doubled, and you have effectively not gained anything new in terms of the audio.