Are you sure the population-area is as large as the entire heightfield?
I see some boundary boxes, one I think is from the heightfield, but also one or two inside that?
What also happened to me once is that I first had a portrait render and used vertical FOV in my camera and later switched to a landscape style render.
Then the vertical FOV setting in my camera didn't match the horizontal setup I had and instances/objects weren't placed in my entire camera frustum.
Do you by accident use a (narrow) vertical FOV?
Are you using an image-map as density shader? If so, is it using the same coordinates as the heightfield? Especially the "centre" or "lower left" setting is important in matching masks.
