In the meantime I wrote to Keyecu (production company of my key) with question about update of firmware of key and description of fail behavior, but without answer. I found video clip for complete assembling of Giulia/Stelvio key:
I could open the key to check possibility of switching from 5- to 3-button function, but think that non compatible firmware is in my key, probably.
Btw, it is spare key and described bad functioning isn't too problematic.
Being Merlin and not Arthur!