WPF RenderTargetBitmap Missing Elements

I have a TreeView with small icons displayed in the data template. I'm trying to save the Treeview as a PNG using RenderTargetBitmap.

The image saves correctly on small data sets. However, if the data set becomes too large, some of the icons are excluded from the final image. The magic number seems to be 200 items. It doesn't seem to matter if the tree is deep or wide, after 200 items, the icons are not rendered.

Added Code

So here is my code that I'm using to create an image.

        RenderTargetBitmap targetBitmap = new RenderTargetBitmap(
            (int)_treeView.ActualWidth,
            (int)_treeView.ActualHeight,
            96, 96, PixelFormats.Default);

        targetBitmap.Render(_treeView);

Added Screen Shot

Notice the missing icons way over on the right side of the tree.

Now if I collapse a few branches, thus hiding some of the other icons, then these icons are included. It's almost like RenderTargetBitmap.Render doesn't have the power to render all of the icons. Or it may have something to do with virtual panels.

Here is a closer look.