Hi, Soul-Burn, and thanks for chiming in…
Since starting this thread, I think I tried everything I could think of, sadly…
I tried to set maxvalue at 16383, 127 and even 1, as the incr/decr value parameter received from my MPD32 is always 1. From this, I also tried each of these settings with the flag value at 0, 8, 16 and even 24 in my MPD32 Generic Remote definition file. Once done I changed every possible flag combination possible from ’ , , ’ to ‘R/T/R’ (0 → 7) in the ‘Device setup…’ window. Took me a while, and I got this way any possible flag values from 0 to 31…
Whenever I save my MPD32 Generic Remote definition, Cubase updates only the 3 last bits of the flagvalue, which means that it keeps the manually setted 4th and 5th bits of it (when manually adding it at a 8, 16 or 24 value). But no matter all the combinations tried, it still doesn’t work.
About the incr/decr format, the MPD32 only send a 5 events block, as following (if the address is set to MSB:000 and LSB:000 in the MPD32) :
C99 → 0
C98 → 0
C96 → 1 (incr) / C97 → 1 (decr)
C99 → 127
C98 → 127
Seems logical to me as the NRPN incr/decr only uses 1 byte (not 2, as a NRPN fine adjustments such as pitch bend, which would use one byte for C38 and another for C6). And, I checked it, it’s the only way the MPD32 can send NRPN controllers messages, sadly.
So, I’m still without solution, here. Would like a feedback from someone in Steiny crew to explain us how we are supposed to set Cubase for actual NRPN messages reception, as I find incredible that a flagship host such as Cubase, with all its supposed MIDI ability, is not able to cope with endless knobs controllers… 
Additionaly, I would like feedbacks from anyone here who uses such knobs to control Cubase, if there are any…
Thanks again !