I think Logic sometimes calls the AU's reset() method instead of note-off, depending on how you are moving the playhead. Just override AUAudioUnit.reset() and reset the state of your AU (including releasing any held notes).

I got a reply from Apple via DTS confirming it's a bug in Logic. To get the host sample rate from for a MIDI plug-in you are supposed to provide an output bus and then read the sample rate through the format property after allocateRenderResourcesAndReturnError called. And Logic doesn't seem to update the format property. If anyone wants to submit a duplicate the bug ID is FB12042397. I've also started a Github project to demonstrate bugs or challenges I come across with AUv3s. https://github.com/Nikolozi/AudioUnitV3Experiments/tree/auv3_midi_sample_rate

Apple started publishing the AU SDK on GitHub as of last year, you can find it here: https://github.com/apple/AudioUnitSDK There's also an examples code repo: https://github.com/apple/AudioUnit-Examples

If I remember correctly, if you have default/initial value set in your DSP state implementorValueProvider will pick it up and you won't need to set it initial value via AUParameter. Another thing to keep in mind is that hosts can set AUParameter to a value that's outside the min/max range. So, be sure to handle that.

If you are trying to do any file access from the audio unit extension, e.g preset import/export, be sure to add the following entry to the extension's entitlements file: <key>com.apple.security.files.user-selected.read-write</key> <true/> Otherwise, you will get exotic crashes when trying to display file access panels (e.g. SwiftUI's .fileImporter()).