Has anybody tried Mountain Mode (MM) while State of Charge (SOC) of battery is high (over 45%) in EV mode?

It is known that if you shift to MM in Charge Sustain (CS) mode engine would top the battery up to 45% SOC as well as providing enough power for current driving power demand.

I am just wondering if we shift to MM when SOC high (say 80%), Does the engine kicks on and sustain the battery charge at (80%) or Volt just drain the battery to designed 45% SOC even in mountain mode.

Imagine I want to save battery charge for city driving at the end of 40 mile highway. So I can shift to MM during highway to save battery charge for using the charge in city instead of highway in which the petrol engine operates efficiently.