Ja consegui, ali onde tinha o título, continha:
<TextBlock Margin="3,-7,10,10" Style="{StaticResource GroupHeaderTextStyle}" Foreground="White" Text="Personagens"
/>
<TextBlock Text="{StaticResource ChevronGlyph}" FontFamily="Segoe UI Symbol" Margin="0,-7,0,10" Style="{StaticResource
GroupHeaderTextStyle}"/>
eu a alterei sem querer, o certo seria:
<TextBlock Text="{Binding Title}" Margin="3,-7,10,10" Style="{StaticResource GroupHeaderTextStyle}" />
<TextBlock Text="{StaticResource ChevronGlyph}" FontFamily="Segoe UI Symbol" Margin="0,-7,0,10" Style="{StaticResource
GroupHeaderTextStyle}"/>
Obrigado.