The observed RadMenu behavior is expected. By design RadMenu in Mobile render mode should have empty space between the last item displayed and the viewport border.
When viewed in Portrait mode, all six menu items can be displayed as there is enough space available between the last item and the viewport border. When viewed in Landscape mode, in order to follow its design, the RadMenu shows only 4 items and the other 2 items can be seen by scrolling the menu items. You could always show all 6 items by setting Menu height through the RadMenu Height property.
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