The owner manual explains that the 4x4 engagement system (which is electronically controlled) 'detects' the presence of the space saver and reduces 4x4 to a minimum. On the road, the car is always in 2WD drive mode (front wheels) anyway (unless you select the ELD function - or 'off-road' setting in Cross -- both are the same setting... something the handbook strongly advises against if on tarmac.)
This text comes from page 60 of the current owner's handbook:
If the space-saver wheel is used
If the space-saver wheel is used, the control unit for 4x4 drive detects its
presence (via the wheel speed signals) and reduces the intervention of the
four-wheel drive mode to a minimum, while guaranteeing the agility of the
vehicle. No fault/alarm signal is displayed on the instrument panel or on
the display.