I looked into this last week. It seems that Logic will use the newer MIDIEventList API if it is available, and fall back to the old MIDIPacketList API otherwise. MIDI FX using the EventList seem to work correctly, but the PacketList is still broken.
I’ve added the following change on develop:
This will use the MIDIEventList when it is available, which should avoid running into the MIDI FX issues that have been reported on Arm. However, this is a workaround, rather than a fix. As far as I can tell, the problems seen when using the MIDIPacketList API are caused by Logic itself. Ideally, Logic should be patched in order to maintain support for existing plug-ins using the old API, rather than forcing plug-ins to be re-built with support for the new API.
Please try updating and see whether this change resolves the issues you’re seeing with MIDI FX AU plug-ins in Logic on Arm.