I don't see the message that you describe when I went to the web page you mentioned. However, I have found a number of comments that are in the header files but not in the online documentation. By the way, if the keyboard is sending the SysEx message in the same packet as the NoteOn, that would appear to be a problem with the keyboard, not CoreMIDI (Apple has enough problems with CoreMIDI without taking on those of everyone else.) By the way, problems with multiple MIDI streams is the reason that real-time and SysEx can't be merged with other types of messages.
The thought occurs to me that CoreMIDI may be trying to merge messages with the same timestamp into the same packet in order to increase performance. As mentioned below, this is illegal with RealTime and SysEx messages.
Do you have a binary dump of the SysEx message. The first few bytes should contain information on which vendor's equipment or DAW that is generating the message.
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