Paul, just to clarify...
You're saying the SD90 is more picky about MIDI timing than ??
And that the PX560 is better as a controller than your old Yamaha S80, correct?

Yes. I should have clarified that the Ketron SD90 (direct MIDI connected with the Yamaha S80) is pickier than software arranger program like vArranger 1.18.04, and arranger related software like V-Console 2.1.4 for Genos (through irig 2 MIDI interface, midiplus 8x8 USB 3.0 MIDI interface) running on Windows 10 laptop, and VST programs such as MainStage 3.3.2 on MacBook Pro (through the MIDI interface of Behringer U-PHORIA UMC404HD) connected with the Yamaha S80.

The old Yamaha S80 occasionally have stuck MIDI notes with Ketron SD90 module, whereas the Casio PX560M has no such problem, both similarly directly connected to SD90 module through SD90's MIDI 2 in port.

However, the old Yamaha S80 works fine with vArranger, V-Console for Genos on Windows, and MainStage 3 on Mac OS X without any stuck MIDI notes.

Thanks,
Paul