When opening the ContextMenu and hovering over the first Item no Tooltip is displayed. Only when you move the mouse over another MenuItem on the same ContextMenu and then back to the first one the Tooltip gets displayed.

As a workaround, the telerik:RadToolTipService.ToolTipContent property can be used:

<telerik:RadMenuItem Header="I should display a tooltip" telerik:RadToolTipService.ToolTipContent="I am the tooltip that never shows" />

The template of the tooltip can also be modified to match the default tooltip style:

    <Style TargetType="telerik:RadMenuItem" BasedOn="{StaticResource RadMenuItemStyle}">
        <Setter Property="telerik:RadToolTipService.ToolTipContentTemplate">
            <Setter.Value>
                <DataTemplate>
                    <Border
                                Background="{telerik:FluentResource ResourceKey=PrimaryBackgroundBrush}"
                                BorderBrush="{telerik:FluentResource ResourceKey=BasicBrush}"
                                BorderThickness="1"
                                SnapsToDevicePixels="True">
                        <ContentPresenter Content="{Binding}" Margin="5"/>
                    </Border>
                </DataTemplate>
            </Setter.Value>
        </Setter>
    </Style>

Available in LIB version 2017.1.410 , it will be also available in the R2 2017 Release. 

Initially, when we open the RadContextMenu and the mouse is over a RadMenuItem1 (that has child RadMenuItems) if we navigate with the keyboard to other RadMenuItem2(that also has child RadMenuItems) and open it (with Enter key for example) - the RadMenuItem2 popup closes because the RadMenuItem1 popup opens after.

If you change the language and the headers of the menu items at runtime, in some cases RadContextMenu is cut and doesn't show full content of the headers.