I just tried out the steps from the linked post with the AudioPluginDemo in Logic 10.7.2, both as AUv2 and AUv3, and native/Rosetta, all under macOS 12.1. (To get the AudioPluginDemo AUv3 to load properly in Logic on Monterey it is necessary to disable the MIDI output due to another Monterey bug). That is, I put the AudioPluginDemo synth on an instrument channel, created a MIDI region with a single long note, and then tried playing the project then seeking to a specific location. Every time I tried, the seek action caused the plugin to receive a note-off matching the active note, followed by an ‘all notes off’ controller change. I did not observe any stuck notes.
I’m seeing a few problems (including unexpected note messages) with MIDI FX AUs in Logic, but I haven’t been able to trigger any stuck notes with a plain synth plugin yet. To investigate further, it would be useful to have the information I mentioned in my previous post, along with a test procedure that triggers the issue.
Edit: Here’s some more info about the MIDI FX issues I was seeing. I was testing using JUCE’s MIDILogger demo, which just passes through all MIDI messages. With two of these plugins in series, I was seeing that the second plugin sometimes logged additional unexpected messages (including note-ons). These always seemed to follow a big block of sysex data, which Logic sends to each MIDI FX plugin after playback starts. It seems that the first plugin in the chain tries to pass this sysex message through to the second plugin, but the message gets mangled in transit. To test this theory, I tried adding a button to the MIDILogger to send an equivalent block of sysex data directly. Now, when I send the sysex block from the first plugin, the second plugin never receives it, but instead receives several garbage messages including note-ons and aftertouch messages. It seems that Logic’s processing of MIDI outputs doesn’t handle sysex properly in 10.7.2. I’m reasonably confident this is a Logic bug; you may be able to work around it by filtering sysex messages from the output of MIDI FX AU plugins.