I couldn’t wait any longer, so I tried something.
I bought a M-Audio MIDISport 1x1 M-Audio and connected it to the Yamaha KX8 I use. Now, if I turn on the keyboard and change the output to MIDI from USB after Cubase has started, all is well. Cubase sees the keyboard and I can record etc. I still lose out though, as the transport controls on the KX8 don’t work over MIDI, just USB.