Fill Volume - gap?

Hello,

Working unit OpenRoads Release 2 Update 8 - 10.08.01.33

There are atached 2 photos of a retention pond, where you will be able to see cut&fill issue.

3D Pond view with components, cut&fill volume.

After turn off grass layer (20cm) it looks like there is a gap in fill volume?

There is no component designed in missing fill volume area.

Is this an error or cut&fill is not developed enough, pond is not a big thing but 10 km of road?

I am worried about the fill volume calculation, could anyone explain it to me?.