Situation: Track "Source" has a MIDI item. Track "Instrument" has a virtual instrument, master send disabled, receives midi input from "Source" and sends audio back ("Feedback..." option is enabled). Everything works as expected, until you add new audio send from "Source" track to, say, track with reverb. Now no midi sent to "instrument" for some reason.
In the attached example project unmute "Send" send to block MIDI send.