I finally got it!
It all depends on the IAT sensor location.
Depending if the sensor is locatet before / after the blower, in or outside the air duct, the fuel trims change.

To discover it I disconnected the MAF forcing the PCM to go in SD: in this particular situation the temperature has a HUGE rule in the fuel calculation (p*V = n*R*T) wink A few Fahrenheit more or less and the fuel trims make a jump.

I think that with wrong IAT values the calculated air mass was a lot different than the air mass reported by the MAF, causing the PCM to jump between SD and MAF. Is that possible?

JR: where do you install the IAT in FI applications?
Theoretically it's better after the blower, in my case it't better to put it close to the air filter: it needs to feel the outside temperature confused This results in very similar fuel trims between SD ans MAF.