Physics2D - The value of Contact2D.normalImpulse is wrong [2018.4.11f1]

There is a falling box hit a another box which stay on ground(box).
See figure 1:

Here are properties of these boxes:

Top Box:

Name: Impacting
Rigidbody2D:

Body Type: Dynamic
Mass: 10
The rests are default.​

Center Box:

Name: Impacted
Rigidbody2D:

Body Type: Dynamic
Mass: 1
Collision Detection: Continuous
Interpolate: Interpolate
Constraints: Freeze Rotation Z​

Bottom Box(the ground):

Name: Ground
Rigidbody2D:

Body Type: Static
The rests are default.​

And I write a script to get the impulse value when Center Box is hit.

Code (CSharp):

1.         private void OnCollisionEnter2D(Collision2D collision) {
2.             if (collision.rigidbody == null) { return; }
3.             m_IsHitted = true;
4.             if (!showConsole) { return; }
5.             Debug.LogFormat("[{0}]OnCollisionEnter2D {1}", s_FixedUpdatedCount, collision.collider.name);
6.             foreach (var contact in collision.contacts) {
7.                 Debug.LogFormat("Normal Impulse: {0}, Hit Point: {1}", contact.normal * contact.normalImpulse, contact.point);
8.             }
9.             Debug.Log("==============");
10.  
11.         }
12.         private void OnCollisionStay2D(Collision2D collision) {
13.             if (collision.rigidbody == null) { return; }
14.             if (!showConsole) { return; }
15.             Debug.LogFormat("[{0}]OnCollisionStay2D {1}", s_FixedUpdatedCount, collision.collider.name);
16.             foreach (var contact in collision.contacts) {
17.                 Debug.LogFormat("Normal Impulse: {0}, Hit Point: {1}", contact.normal * contact.normalImpulse, contact.point);
18.             }
19.             Debug.Log("==============");
20.         }

and then the result is:

​

Center Box get (0.0, 109.7f) impulse from Bottom Box(ground)
and get (-70.5f) impulse from Top Box.
It's weird. The total of them must be close to (0, 0) because the original and terminal velocity of the Center Box are close to (0, 0).

and here is the value of inspector:

same.

and I found that if both of Top Box and Center Box are Freeze Rotation or Don't Freeze Rotation then the result will be right.