This is expected behavior. As explained in the Design Considerations for HTML Rendering
"In HTML report items are located in different cells. If you have overlapping items the result of these items would be undetermined i.e. it would break the original report layout. In case of two or more items that have exactly the same locations only the first item is rendered, the others would not be rendered at all. In general you should not overlap report items at design-time if you plan to export the report to HTML. If there are no overlapping items the report layout is preserved."
All the best,
the Telerik team