I have these styles...

<Style x:Key="StandardTextStyle" TargetType="{x:Type TextBlock}">
    <Setter Property="FontSize" Value="20" />
    <Setter Property="Foreground" Value="{StaticResource ControlForegroundBrush}" />
    <Setter Property="FontWeight" Value="Regular" />
    <Setter Property="HorizontalAlignment" Value="Center" />
    <Setter Property="VerticalAlignment" Value="Center" />
</Style>

<Style x:Key="MeasurementTextStyle" TargetType="{x:Type TextBlock}" BasedOn="{StaticResource StandardTextStyle}">
    <Setter Property="FontSize" Value="28" />
</Style>

When rendering this XAML to the screen, the FontSize is correct (28)

                    <TextBlock 
                        Text="{Binding LetterLabel}" 
                        Margin="2"
                        Style="{StaticResource MeasurementTextStyle}"/>
                    <TextBlock 
                        Text="{Binding FormattedResultInCurrentUnits}" 
                        Margin="2"
                        Style="{StaticResource MeasurementTextStyle}"/>
                    <TextBlock 
                        Text="{i18n:TranslateBinding Path=UnitsLabel}" 
                        Margin="2"
                        Style="{StaticResource MeasurementTextStyle}"/>
                </StackPanel>

But when rendering offscreen to save as a file, the FontSize is not correct (20)