Place yourself down and one subordinated named u1.
Create an empty Marshall and name it apc1.
Preview the mission.
In the debug console type u1 assignAsTurret [apc1,[0]]; [u1] orderGetIn true;
Nothing will happen.
Checking u1's assignedVehicleRole show it has been assigned properly.
Now do [u1] join grpNull. and the unit will make a beeline for the apc and mount it properly according to assigned turret. Same thing happens if using a different assign command e.g assignAsGunner, assignAsDriver
Manuallly selecting u1 and pointing at the apc makes him load up in the assigned position which makes it look like the script command orderGetin is at fault.