Works ok here (build 316b). Was able to drag the (midi) melody track from either of two different mgu's to RB. With or without plugin mode activated.

You might try it with RB. Perhaps there's an issue with your default DAW? Although can't explain the apparent version specificity.

Other thoughts: check that the pref settings for DnD are proper. Are midi files showing up in the DnD folder?

-Ron