Bug in Grouping

I have a ListView which is grouped by the first letter in the string. Many of the letters work fine, others do not appear, or take a while to appear. Is this a known bug, or am I doing something wrong?

Here is the top of my ListView and attached is an image of the list

            <ListView
                x:Name="DirectoryList"
                ItemsSource="{Binding MembersGrouped}"
                ItemTapped="OnItemSelected"
                IsGroupingEnabled="True"
                GroupDisplayBinding="{Binding Key}"
                GroupShortNameBinding="{Binding Key}"
                RowHeight="37"
                HasUnevenRows="true">
                <ListView.GroupHeaderTemplate>
                    <DataTemplate>
                        <ViewCell>
                            <StackLayout
                                VerticalOptions="FillAndExpand"
                                Padding="{x:Static common:PaddingResources.MainBodyPadding}"
                                BackgroundColor="{x:Static common:ColorResources.SubHeader}">
                                <Label
                                    Text="{Binding Key}"
                                    TextColor="White"
                                    VerticalOptions="Center" />
                            </StackLayout>
                        </ViewCell>
                    </DataTemplate>
                </ListView.GroupHeaderTemplate>