I guess they just abandoned the idea of moving AT gun and instead put the resources into the others
priority objectives 
I hope not.
Remember that AI stances were announced… in antient times lets say and they were released not long ago.
The only usefull stance is "passive".
Because there is no “follow me. I said follow me! Why the fuck are standing in the middle of a field while you try to shoot at an ennemy on the other side of the map?! Take cover at least i don’t! You know what? Just die you useless piece of shit” order for your squad member.
But AT guns needs wheels to move. Oh wait…
And don’t forget the current AT gun has a driver’s seat, i wonder why 
I will give my vote for this command.
Nothing like trying to get your squad stay in trench only to be mowed down by anything seconds later. Just because AI decided to climb out of trench to enjoy sunlight.