Xamarin Problemas de Grid e Sobreposição no iOS

ContentPageGridOK.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:d="http://xamarin.com/schemas/2014/forms/design"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             mc:Ignorable="d"
             Shell.NavBarIsVisible="True"
             BackgroundColor="{StaticResource StrongBGColor}"  
             xmlns:vm="clr-namespace:DriverPerformance.ViewModels"
             Title="{Binding Title}"
             Visual="Material"
             x:Class="DriverPerformance.Views.Comprovantes">

    <ContentPage.BindingContext>
        <vm:ComprovantesViewModel />
    </ContentPage.BindingContext>

    <ContentPage.ToolbarItems>
        <ToolbarItem Text="Filtros" Clicked="Filtros_Clicked"/>
    </ContentPage.ToolbarItems>

    <ContentPage.Content>
        <Grid VerticalOptions="Fill" Padding="15,10">
            <ListView ItemsSource="{Binding Comprovantes}"
                      VerticalOptions="Fill"
                      x:Name="ListViewComprovantes"
                      SelectionMode="Single"
                      IsPullToRefreshEnabled="True"
                      Refreshing="ListViewComprovantes_Refreshing"
                      ItemSelected="ListViewComprovantes_ItemSelected"
                      RowHeight="65">
                <ListView.ItemTemplate>
                    <DataTemplate>
                        <ViewCell>
                            <Grid BackgroundColor="{StaticResource LightBGColor}" Margin="0,5" Padding="8,5">
                                <Grid.ColumnDefinitions>
                                    <ColumnDefinition Width="*" />
                                    <ColumnDefinition Width="Auto" />
                                </Grid.ColumnDefinitions>
                                <Grid Grid.Column="0">
                                    <Grid.RowDefinitions>
                                        <RowDefinition Height="Auto" />
                                        <RowDefinition Height="Auto" />
                                    </Grid.RowDefinitions>
                                    <Label Grid.Row="0" Text="{Binding DataRegistro, StringFormat='{0:dd/MM/yyyy}'}" VerticalOptions="Start" FontAttributes="Bold" />
                                    <Label Grid.Row="1" Text="{Binding TipoComprovante}" VerticalOptions="End" />
                                </Grid>
                                <Label Grid.Column="2" 
                                       Text="&#xf138;"
                                       FontFamily="FontAwesomeBold"                                       
                                       FontSize="24"
                                       Margin="5,0,2,0"
                                       VerticalOptions="Center"
                                       HorizontalOptions="End" />
                            </Grid>
                        </ViewCell>
                    </DataTemplate>
                </ListView.ItemTemplate>
            </ListView>
          <!-- O Botão aparece normalmente sobrepondo o listview no canto inferior esquerdo da pagina -->
            <Button Text="&#xf067;" 
                    FontSize="24"
                    x:Name="btnNovoComprovante"
                    Clicked="btnNovoComprovante_Clicked"                    
                    FontFamily="FontAwesomeBold"
                    VerticalOptions="EndAndExpand" 
                    HorizontalOptions="EndAndExpand" />
            <Frame Padding="10"
                   CornerRadius="35"
                   Margin="10"
                   Opacity="0.8"
                   BackgroundColor="{StaticResource StrongBGColor}"
                   IsVisible="{Binding IsBusy}"
                   VerticalOptions="StartAndExpand"
                   HorizontalOptions="CenterAndExpand">
                <ActivityIndicator x:Name="actInd"
                                   HeightRequest="50"
                                   WidthRequest="50"
                                   VerticalOptions="CenterAndExpand"
                                   HorizontalOptions="CenterAndExpand"
                                   BackgroundColor="{StaticResource LightBGColor}"
                                   IsRunning="{Binding IsBusy}"
                                   Color="{StaticResource PrimaryColor}"/>
            </Frame>

        </Grid>
    </ContentPage.Content>
</ContentPage>

ContetPageGridBad.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:d="http://xamarin.com/schemas/2014/forms/design"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             mc:Ignorable="d"
             Shell.NavBarIsVisible="True"
             BackgroundColor="{StaticResource StrongBGColor}"  
             xmlns:vm="clr-namespace:DriverPerformance.ViewModels"
             Title="{Binding Title}"
             Visual="Material"
             x:Class="DriverPerformance.Views.ComprovanteItem">

    <ContentPage.BindingContext>
        <vm:ComprovanteItemViewModel />
    </ContentPage.BindingContext>

    <ContentPage.Content>
        <Grid VerticalOptions="Fill" Padding="15,10">
            <ScrollView VerticalOptions="Fill">
                <StackLayout VerticalOptions="StartAndExpand">
                    <Label Text="Tipo de Comprovante" FontAttributes="Bold"/>
                    <Picker ItemsSource="{Binding Tipos}" SelectedItem="{Binding TipoComprovante}" IsEnabled="{Binding PodeSalvar}" />
                    <Image Source="{Binding ComprovanteSource}" />
                    <Grid HeightRequest="60"></Grid>
                </StackLayout>
            </ScrollView>
          <!-- Os botões ficam por traz do shell na versão de iOs  -->
            <Grid VerticalOptions="EndAndExpand">
                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="Auto" />
                    <ColumnDefinition Width="Auto" />
                    <ColumnDefinition Width="*" />
                </Grid.ColumnDefinitions>

                <Button Grid.Column="0" 
                        Text="&#xf0c6;"
                        FontSize="20"
                        WidthRequest="80"
                        HorizontalOptions="Start"
                        x:Name="btnAnexar"
                        Clicked="btnAnexar_Clicked"
                        FontFamily="FontAwesomeBold" />
                <Button Grid.Column="1" 
                        Text="&#xf030;"
                        FontSize="20"
                        WidthRequest="80"
                        x:Name="btnFoto"
                        Clicked="btnFoto_Clicked"
                        HorizontalOptions="Center"
                        FontFamily="FontAwesomeBold" />
                <Button Grid.Column="2" 
                        x:Name="btnSalvar"
                        IsVisible="{Binding PodeSalvar}"
                        Clicked="btnSalvar_Clicked"
                        HorizontalOptions="End"
                        Text="Salvar" />
                <Button Grid.Column="2" 
                        x:Name="btnExcluir"
                        IsVisible="{Binding PodeExcluir}"
                        Clicked="btnExcluir_Clicked"
                        HorizontalOptions="End"
                        Text="Excluir" />
            </Grid>

            <Frame Padding="10"
                   CornerRadius="35"
                   Margin="10"
                   Opacity="0.8"
                   BackgroundColor="{StaticResource StrongBGColor}"
                   IsVisible="{Binding IsBusy}"
                   VerticalOptions="StartAndExpand"
                   HorizontalOptions="CenterAndExpand">
                <ActivityIndicator x:Name="actInd"
                                   HeightRequest="50"
                                   WidthRequest="50"
                                   VerticalOptions="CenterAndExpand"
                                   HorizontalOptions="CenterAndExpand"
                                   BackgroundColor="{StaticResource LightBGColor}"
                                   IsRunning="{Binding IsBusy}"
                                   Color="{StaticResource PrimaryColor}"/>
            </Frame>

        </Grid>
    </ContentPage.Content>
</ContentPage>