The vanilla arm-swinging animation causes the player's body to rotate to match their head, however performing a weapon attack does not do so. Since the hitbox of a weapon attack is based on the head's rotation, this can cause weird-looking behaviour, especially with narrow attacks like the Trident's thrust attack - if the body is facing straight north, but the player's head is turned northeast, the animation will point north but the actual hitbox will point northeast. Forcing the body to match the head's yaw while attacking would fix this.

What a find!