I've reviewed the code in question again and it still holds. Audacity returns DROPEFFECT_MOVE at drag and drop without any modifier keys pressed and DROPEFFECT_COPY when Ctrl is pressed.
When the target app of the drag and drop reports that files are moved, I have no other chance than removing the files from the file list. I have no indication of where the files are moved to and would end up with orphaned files on actual moves.
One option to circumvent these problems would be to disallow move operations via drag and drop from Mp3tag all together. However, this would probably and most likely disappoint people who are using the feature as intended.
Once again, I'm moving this to #bug-reports:no-bugs