I found a workaround resolution to the problem...
I created a RadMenu.skin file (or you could add the style manually to each RadMenu) with the following:
Note the style="z-index: 1" seems to have resolved the problem. I tried the RadMenu ZIndex property but it did not seem to work. I also tried a z-index of 100, but that made RadComboBox selections display behind the RadMenu.
Hope this helps.