This is how combat worked in vanilla… Partially.
In vanilla, combat with dungeon mobs was an aura effect. This meant that if you got too close to anyone who was in combat, or any mob that was in combat, it would pull you into combat. “Active” buffs, such as auras and active hots, would also engage the casting party member into combat if someone buffed engaged. So if a paladin had devo aura up, and you face pulled, the paladin 20 yards away would be plunged into combat. Same thing if a priest had a renew ticking on you and you blundered into a pack of mobs.
This is all working as intended and exactly how it used to be.
However.
This was not how combat in the world used to work. In the overworld, combat used to work exactly as it does today. You could stand ontop of a mob and player fighting, but would not be engaged in combat unless you engaged them, or were hit by aoe (mobs that cleave for example). Also, I’m fairly sure that hitting the player wouldn’t put you in combat with the mob.
The problem is that, currently, we have the dungeon system… everywhere. So combat is an aura / aoe effect in the open world, which is not how it used to be.