The density at viewport is not indicative of the rendering result, because the number of items is limited by Display->Viewport->Max Items.
Please, decrease the density of Distribution map until the error dissapear. Then adjust the density using the render as guide, to get the desired result. In any case, you would increase the limit of items at render (actually 500.000) setting Display->Render->Max items, but probably this is not necessary.
Alternatively, if you want to adjust the density visually, create a smaller grass region (for example 2 m x 2 m) and test different settings there. Once you get a nice result, apply it to the full area.