After the mortars start falling weird stuff happens.
First of all, my AI squad has decided to walk through the mortar fire.
Second, the main issue: Mortars behave like guided missiles. They can hit you and track you with increasing accuracy, even though you're running at full speed.
Seems to be more likely if you run away solo from the squad, but basically after a point the chances of you getting hit are 100%.
Considering that mortars have difficulty hitting even static targets, the fact that they can hit (and track accurately) a sprinting soldier is hard to believe.
Summary: Mortars track you specifically, and become increasingly accurate, to the point when your getting killed is certain.