Hi Tobias,

I am sorry to hear this important to you functionality is still not available.
Since the issue is not reproducible with the old “Nested” mode, may I ask you to try not setting GroupRenderMode=”Flat” (i.e. using the default GroupRenderMode=”Nested”)?

For example:
<telerik:RadGridView x:Name="PeopleGrid" ItemsSource="{Binding People}" AutoExpandGroups="False">
            <telerik:RadGridView.GroupRowStyle>
                <Style TargetType="telerik:GridViewGroupRow" BasedOn="{StaticResource GridViewGroupRowStyle}">
                    <Setter Property="IsExpanded" Value="{Binding Converter={StaticResource SomeConverter}, FallbackValue=True}" />
                </Style>
            </telerik:RadGridView.GroupRowStyle>
</telerik:RadGridView>