Maybe iOS 14 has the habit to merge several MIDI messages into a single packet (which is illegal for SysEx messages), and your app has a problem with parsing these packets?

There are no SysEx messages involved? I have problems only with these, but no issues with multiple channels. To be more specific: The user plays a chord on the keyboard. The keyboard adds a SysEx message after the third note, to signal it has detected a chord. The app ought to receive four packets: Note On Note On Note On SysEx (chord detection) But it receives three packets: Note On Note On Note On, SysEx This is illegal, as https://developer.apple.com/documentation/coremidi/midipacket?language=objc says "In the case of system-exclusive messages, a packet may only contain a single message, or portion of one, with no other MIDI events." and my apps are not prepared to handle these malformed packets. Your problem seems to be a different one, though.

Did you inspect the incoming MIDI packets? My impression is that iOS 14 does not handle SysEx messages correctly. I have a case where a SysEx messages arrives after a Note On message, in the same packet. This does not conform to the Core MIDI specification.