See this Screencast for a short video of the issue at runtime: https://www.screencast.com/t/YXm6xOtO 

Here's the code to reproduce: 

<grid:RadDataGrid ItemsSource="{x:Bind ViewModel.DataItems, Mode=OneWay}"
                  UserGroupMode="Disabled"
                  UserFilterMode="Disabled"
                  UserColumnReorderMode="None"
                  UserSortMode="None"
                  AutoGenerateColumns="False"
                  GridLinesVisibility="Horizontal">
    <grid:RadDataGrid.Columns>
        <grid:DataGridNumericalColumn Header="1"
                                      PropertyName="Key"
                                      SizeMode="Auto" />
        <grid:DataGridTextColumn Header="2"
                                 PropertyName="First"
                                 SizeMode="Auto" />
        <grid:DataGridTextColumn Header="3"
                                 PropertyName="Second"
                                 SizeMode="Auto" />
        <grid:DataGridTextColumn Header="Stretchy"
                                 PropertyName="Stretch"
                                 SizeMode="Auto" />
        <grid:DataGridTextColumn Header="5"
                                 PropertyName="SecondLast"
                                 SizeMode="Auto" />
        <grid:DataGridTextColumn Header="6"
                                 PropertyName="Last"
                                 SizeMode="Auto" />
        <grid:DataGridNumericalColumn Header="7"
                                      PropertyName="Count"
                                      SizeMode="Auto" />
    </grid:RadDataGrid.Columns>
</grid:RadDataGrid>