Vector3.Angle outputs different results for the same values

I've been using Vector3.Angle to determine the angle at which a ball hits a wall.

   private void OnCollisionEnter(Collision collision)
         {
                 Vector3 normal = collision.contacts[0].normal;
                 Vector3 velocity = GetComponent<Rigidbody>().velocity;
      
                 Debug.Log("Normal: " + normal + "Velocity: " + velocity + "Angle:" + 
                 Vector3.Angle(velocity, -normal));
         }

Even though 'Normal' always remains at (0.0, 0.0, -1.0) and 'Velocity' at (0.0, 0.0, 0.0) when the ball hits the wall along its normal the 'Angle' will sporadically output 180 or 90 sometimes. Any idea why or how to fix?