So I figured maybe it has to do with USB Midi. I disconnected the USB cable and connected up a midi lead and… Problem solved.

Really surprised by this. I would imagine that USB would be more solid than just Midi but clearly not. I wonder if this is an A4 issue or USB Midi issue?

Only problem I have now is not enough Midi channels - transmitting via USB freed them up.

If it’s not one problem it’s another.