Hi markbee,
please do the following tests, and then come back with the test results:
1. Press the "UPPER OCTAVE - (minus)" button once.
>>> Is the "much louder" key still D3 then, or is D4 now the "louder" key?
2. Press "UPPER OCTAVE +/-" buttons simultaneously to come back to the normal state.
3. Play the D3 key and some keys below / above D3 as SOFT as possible.
>>> Is D3 playing always with full Velocity (so that it makes no difference if you hit the D3 key very soft or very hard)?
Best regards,
Chris