I’ve got three identical 8-port MIDI interfaces (MOTU MIDI Timepiece AV USB). Sometimes, when Cubase sends MIDI data to a MIDI port on an interface, it replicates the stream to every MIDI output port of the interface. This is indicated by the MIDI activity LEDs on the interface’s front panel. So, for example, if I’m routing a MIDI Track to a MIDI Device assigned to Interface #1, Port #1, I might see activity on all ports (1-8) of Interface #1. But not always. Sometimes, I’ll see activity only on Interface #1, Port #1, as would normally be expected. Why is this happening?
Choosing another destination for the MIDI Track sometimes makes the problem go away. Then, I can go back to the original destination MIDI Device and everything’s fine - at least, for a while. But it’s random. I can click through a dozen MIDI Devices and MIDI Ports with no effect, but then, suddenly, it’ll work and the problem will go away. When restarting Cubase and repeating the exact same steps, sometimes the problem will crop up on a MIDI Track. Other times, it won’t.
The issue is isolated to each interface so that, for example, if I get it fixed on Interface #1, the other two will continue to exhibit the problem until I get those fixed as well. Likewise, which interfaces are affected to begin with is also random.