Can't overdub on MIDI items if imported as MIDI reference

[dpbsound]

When I import MIDI files as a reference (as set in the Preferences), I can't seem to get MIDI Overdub to work. MIDI overdub works fine for any MIDI items already in the project, but if they are imported as a reference, no changes are made to the file. This seems odd because I can still go in and manually add MIDI notes and CC data but doing this live via Overdub doesn't work.

I need to keep the MIDI items set as reference.