I would like to clarify a couple of things about your scenario. An expected behavior is applying a higher z-index value to the currently expanded menu item in order to get that item to top position. In this case if the currently expanded menu item is overlapping any other item might cause such appearance behavior showed in the provided screenshot.
An easy and convenient approach to avoid this issue would be to use one RadMenu instance and add hierarchical structure of all items instead of using a single RadMenu for each root item. This way you can avoid that overlapping issue and any conflict consequences that might occur.
the Telerik team
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