To reproduce: Run AU Instrument example code provided by Xcode. When it asks which application to run in, select Logic. Create an instrument track, and set the instrument to be the AU Instrument Create a region in the track and add a note Set the playhead just before the note and press the play button Click ahead of the note (just after the note ends) Result: the AU Instrument is still playing Expected result: The AU Instrument stops playing (i.e. receives the note off event)

For some reason, Logic sends a Program change (bank, program, options) MIDI message type to the AU Instrument whenever the playhead is moved. Though not ideal, I'm experimenting with using this message to turn off all notes currently playing. Reasoning: if I changed the current program on a physical synthesizer, I'd expect the sounds that are currently being played to stop (since the new patch sounds different than the old patch)