Thanks for analyses, i never carry about sonar/rangefinder param since i removed the analog one 2 years ago... I think you gave me the answer, and will confirm that tomorrow, I planned a testing/tuning session. It's just strange that this issue appeared only with 3.4 upgrade as I didnt changed anything relative to rangefinder since a long time.
For the motor spin, I put 0 to have a static props when landed. As it is high on it's legs with batteries on top and good power set up, a little thrust+wind could make it fall. With that "no disarm" issue, i'd rather totally stop the props when throttle down/stab mode.
I keep you in touch tomorrow!