Oké, omdat ik zelf hier de fout in ben gegaan door over een stukje manual heen te lezen, en wellicht andere mensen er iets aan kunnen hebben, hier de oplossing, die, achteraf dus gewoon letterlijk in de manual staat

... (En @Arnomusicman; het klopte inderdaad wat je zei!)
Ondanks dat ik door mijn onoplettendheid enigzins onnodig hier een vraag heb gesteld, hoop ik dat het toch gewaardeerd word als ik voor mensen die tegen eenzelfde probleem aanlopen, de uitgebreide en volledige oplossing hier deel

....
The MIDI messages generated on the KX (by
playing the keyboard, controlling the Pitch Bend
Wheel and the Modulation Wheel, and so on) are
transmitted via USB-MIDI Port 1 from the KX to
Cubase. Keep in mind that MIDI data transmitted
via the USB-MIDI Ports 2 and 3 are not related to
music performance (including the instrument
change). This means that any messages received
over these ports may be recognized as instrument
change and hence may result in unexpected
sounds. To avoid this, you should remove the
checkmark of “All MIDI Input” for USB-MIDI Port 2
and Port 3.
1. Start Cubase on your computer.
Important!
Cubase cannot recognize the KX if you turn the
power of the KX on after starting Cubase. Make
sure to start Cubase after turning on the power of
the KX.
2. Select “Device Setup” from the “Device”
menu of Cubase.
The Device dialog appears.
3. Click “MIDI Port Setup” in the Devices
column.
The MIDI ports of Cubase appear at right of the
dialog.
4. Remove the checkmarks by clicking the
check boxes of “All MIDI Inputs”
corresponding to the KX MIDI Ports 2 and 3
respectively.
“Yamaha KX-2” and “Yamaha KX-3” on
Windows and “YAMAHA KX Port 2” and
“YAMAHA KX Port 3” on Macintosh correspond
to MIDI Port 2 and MIDI Port 3 respectively.
MIDI Ports 2 and 3 of the KX handle the
messages generated via the operations other
than keyboard performance such as operations
via the CONTROL knobs and operations on the
KX Editor of the computer. Removing the
checkmarks corresponding to these MIDI ports
will avoid accidentally recording non-keyboard
data to a track in Cubase.
5. Click [OK] to close the dialog.
NOTE The checkmark of “All MIDI Inputs”
corresponding to the KX MIDI Port 2 should
be removed since “Extensions for Steinberg
DAW” is included normally in “TOOLS for
KX.” Remove the checkmark of “All MIDI
Inputs” corresponding to the KX MIDI Port 2
only if the checkmark is checked for some
reasons.
Want, de 3 MIDI ports van de KX zijn namelijk voor de volgende functies:
MIDI ports on the KX
For data transfer via a USB cable between the
KX and the computer, you can use three MIDI
ports by installing the USB-MIDI driver. Each of
three MIDI ports can handle the different MIDI
messages as described below.
MIDI Port 1:
This port is indicated as “Yamaha KX-1” on
Windows and “YAMAHA KX Port 1” on
Macintosh. This port is used for data transfer of
the musical performance such as keyboard
performance (note on/off), Modulation wheel
operation (Control Change) and Pitch Bend
wheel operation.
MIDI Port 2:
This port is indicated as “Yamaha KX-2” on
Windows and “YAMAHA KX Port 2” on
Macintosh. This port is used for data transfer
generated by using the DAW REMOTE buttons
(with the exception of the Cursor buttons, [ESC]
button and [ENTER] button) and CONTROL
knobs.
MIDI Port 3:
This port is indicated as “Yamaha KX-3” on
Windows and “YAMAHA KX Port 3” on
Macintosh. This port is used for data transfer
generated by operating the KX Editor on the
computer.