Вам понадобится пара изменений.
a) В вашей основной сетке
<Grid Grid.IsSharedSizeScope="True">
b) В панели управления прибором, сетке, удалении сетки. IsSharedSizeScope
<Grid Grid.Row="0" x:Name="InstrumentOperationsPanel">
c) В BuiltInUserActionPanelForQX200 Grid удалите Grid.IsSharedSizeScope
<Grid x:Name="BuiltInUserActionPanelForQX200" Grid.Row="2" VerticalAlignment="Top">
Весь ваш Xaml не будет выглядеть как
<Grid Grid.IsSharedSizeScope="True">
<Grid.RowDefinitions>
<RowDefinition Height="Auto"></RowDefinition>
<RowDefinition Height="*"></RowDefinition>
<RowDefinition Height="Auto"></RowDefinition>
</Grid.RowDefinitions>
<Grid Grid.Row="0" x:Name="InstrumentOperationsPanel">
<Grid.ColumnDefinitions>
<ColumnDefinition SharedSizeGroup="A"></ColumnDefinition>
<ColumnDefinition SharedSizeGroup="A"></ColumnDefinition>
<ColumnDefinition SharedSizeGroup="A"></ColumnDefinition>
<ColumnDefinition SharedSizeGroup="A"></ColumnDefinition>
</Grid.ColumnDefinitions>
<Button x:Name="PrimeButton" Grid.Column="0" Content="ABCD"/>
<Button x:Name="FlushButton" Grid.Column="1" Content="ABCDEFGHIJKL"/>
<Button Content="ABCD" Grid.Column="2"/>
<Button x:Name="PrimeFlush3TimesButton" Grid.Column="3" Content="ABCD"/>
</Grid>
<Separator Grid.Row="1" HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
</Separator>
<Grid x:Name="BuiltInUserActionPanelForQX200" Grid.Row="2" VerticalAlignment="Top">
<Grid.ColumnDefinitions>
<ColumnDefinition SharedSizeGroup="A"></ColumnDefinition>
<ColumnDefinition SharedSizeGroup="A"></ColumnDefinition>
<ColumnDefinition SharedSizeGroup="A"></ColumnDefinition>
<ColumnDefinition SharedSizeGroup="A"></ColumnDefinition>
</Grid.ColumnDefinitions>
<Button x:Name="DRDiagnosticsButton" Grid.Column="0"
Content="ABCD"></Button>
<Button x:Name="QXFieldAgentButton" Grid.Column="1"
Content="ABCD"></Button>
<Button x:Name="QX200MTSButton" Grid.Column="2"
Content="ABCD"></Button>
<Button x:Name="QX200MTSDetectorButton" Grid.Column="3"
Content="ABCD"></Button>
</Grid>
</Grid>
Вывод