This appears to be due to overlapping geometry previews. Select the group, then right click the workspace background and select ‘hide all geometry preview.’ Then select just the GeometryColor.ByColorRange node, right click on it, and enable the preview.
If this doesn’t resolve the issue get even more aggressive with the disabling of geometry - turn all geometry previews off except the last GeometryColor.ByColorRange node. If that fails post the graph and all files required to run it (the rvt, associated links, etc).