DepthTexture doesn't look good after plane clipping

I found that I’m not getting an accurate depth texture after I used clipping just like SimpleWaterProcessor
So I tried TestSimpleWater Example to see if I get the same result and I got it, so what is the solution here?

As you can see in the pics I provide it doesn’t matter how far the camera is from the object, but it matter if the object lower or higher of the frustum of camera

What’s wrong with it?

Depth texture supposed to get lighter of how far from the camera is, which it happened correctly with none clipping approach, but when I use clipping strange behavior happened, no matter how far you are from the object it doesn’t get lighter, it remain solid black
These pics without clipping

and those with clipping