Well, after some investigation, it appears the D-50 sends MIDI CC #123 (all notes off) whenever I release a key whereas the P-85 sends a simple NOTE OFF MIDI message upon releasing a key.
Why is this only causing a problem with Mainstage and not Logic itself (or any other DAW app/softsynth for that matter)? Is there a way to filter out MIDI CC #123 in Mainstage?