Hi all,

I'm pleased to report that I've finally figured it out!

The problem was related to the PortMIDI 2.0.2 library bundled with my Csound 6.18 installation. I tested that library independently, outside Csound, and reproduced the problem: successive MIDI events weren't being delivered correctly to macOS CoreMIDI.

I then tested PortMIDI 2.0.8 (installed via Homebrew), which worked perfectly. Replacing the bundled libportmidi.dylib with that version resolved the problem, and Csound's MIDI output is now working as expected.

So the issue wasn't with my CSD or IAC Driver, but with the bundled PortMIDI library in my installation.

Thanks again to Øyvind and Art for taking the time to help!

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