AI solder engage the invisible and invulnerable gunner of HMGA/GMGA after the gun is disabled. Also you can see the blood splatters on what would be the torso of the gunner if he were visible.

This also means that the AI is wasting shots on the gunner rather than targeting the gun it self, the invis gunner is also acting as a bullet trap protecting the gun itself from certain angles.

I suggest either killing and/or deleting the gunner or setting the gunner as captive when the gun is disabled or out of ammo (not sure if you can rearm an autonomous static).