Mono (Retrig) setting not working on NL4R

[Heliophile]

I'm playing my Nord Lead 4R through an M-Audio Axiom 25 keyboard controller. Whether I connect the keyboard directly to the NL4R using a MIDI cable, or use MIDI over USB through my DAW (Reason), the Mono setting (every note played should retrigger the amp envelope, page 20 in the manual) doesn't work. Instead, the synth behaves as if it's in monophonic legato mode.

The NL4R may not be the cause of the problem, because my Vermona Mono Lancet won't go into mono retrig mode either. That would lead me to believe that the M-Audio Axiom may be the culprit. However, with that same controller, the soft synths in Reason work as expected in mono retrig mode!

Does anybody have an idea what I could be overlooking? I've searched the M-Audio's manual, but it's sooooooo difficult to understand and nothing came up when I searched for 'legato' or 'retrigger'.

[Heliophile]

Update: in the latest manual it's on page 22: "Mono makes the Slot monophonic, only one note at a time will be heard. The envelopes will be re-triggered for each new note you play."

This is not how my NL4 behaves. The envelopes are not retriggered, unless the envelope of the note held down first is fully complete. Only then is there a difference in behaviour between Mono and Legato mode.