Devo10 / Naze32 / Cleanflight calibration problem

I'm new to Deviation (and multirotors) and have trouble calibrating (?) my radio for a naze32 board flashed with cleanflight.

My setup :
- Devo10 (white) flashed with Deviation 4.0.1
- Orange R615X receiver, PPM mode
- Naze32 rev5 10DOF board flashed with cleanflight 1.10.0 (1.11.0 simply want to kill my tricopter ^^)

The radio is properly calibrated in Transmitter menu => Transmitter config => Calibrate.
Channel monitor gives me perfectly centered channel outputs and stick inputs (as far as I you can tell with the low screen resolution).
All channels mixers are set as simple, -150 to 150 min/max, 100% scale, no offset, no expo, no subtrim.
All trim are centered.

Now, connecting everything and using the cleanflight GUI to setup everything, I change the scale+ and scale- for each channel on the Devo to get a nice and clean 1000-2000 (Model menu => Mixe => xxx => Scale+/-).
I then change the subtrim to get a 1500 neutral value... and fail to do so. The subtrim has to be changed quite a lot (2 or 3) before the output value starts to change in cleanflight, but then it does, it just jump from 1492 to 1509 Below 1492 or above 1509, the subtrim allow very fine adjustments (+/ 1 or 2) but there is a kind of "black hole" around 1500 !

My others radios/transmitters (old no name 72Mhz , Flysky i6) does not have this problem with the same Naze32 board.
Did not try with the Devo10 & original walkera firmware.

Any idea on the reason why this happen ?
Maybe I'm missing a step in the configuration ?

Thanks !

Did you ever figure out the issue? I have the same problem!