Yeah rams should be aggressive by default. I’ve noticed the pathing bugs out when the enemy castle fires when I order attack move in defensive stance. They’ll also attack siege weapons attacking them/nearby ones (when aggressive)
Defensive stance changes the behavior of Mangonels so that they don’t hurt friendly units, they’ll only attack if there is a clean shot. On aggressive stance, Mangonels will mow down your own army.
Rams on aggressive mode will chase other siege and towers. Under pathing rules, if a ram can’t path directly to its target (say a castle) it can become insubordinate and attack other nearby things. The ram ends up yolo chasing other siege and you’re left to wonder why the castle is still standing. Defensive keeps them prioritizing the targeted building.
It’s not a bug but yeah there’s no option to change the preset either.