Wp 8.1 xaml issue - xaml

Question

I have TemplatedControl defined as:
<Style TargetType="controls:HeaderedContentControl">
<Setter Property="HintAlignment" Value="Center"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="controls:HeaderedContentControl">
<Grid Background="{TemplateBinding Background}">
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition/>
</Grid.RowDefinitions>
<Border Background="Gray">
<TextBlock Text="{TemplateBinding Header}"
FontSize="25"
Foreground="White"
TextWrapping="Wrap"
Margin="0,10"
HorizontalAlignment="Center"
VerticalAlignment="Center"/>
</Border>
<ScrollViewer Grid.Row="1">
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition/>
</Grid.RowDefinitions>
<TextBlock Visibility="{Binding Text, RelativeSource={RelativeSource Self}, Converter={StaticResource strConv}}"
Text="{TemplateBinding Hint}"
TextAlignment="{TemplateBinding HintAlignment}"
TextWrapping="Wrap"
Foreground="Gray"
FontSize="17"
Margin="10"/>
<ContentPresenter Content="{TemplateBinding Content}"
Grid.Row="1"/>
</Grid>
</ScrollViewer>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
Sometimes after navigation to the page with this control I see
this
instead of
this
I think this occurs because of BottomAppBar on the page. How can I get round this issue?
Layout where this control is used:
<controls:HeaderedContentControl Header="Регистрация">
<StackPanel Margin="10,40">
<TextBlock Text="Номер телефона:"
FontSize="20"/>
<Grid Margin="0,15">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition/>
</Grid.ColumnDefinitions>
<TextBlock Text="8"
Margin="10,0"
VerticalAlignment="Center"
FontSize="20"/>
<controls:PhoneTextBox Value="{Binding PhoneNumber, Mode=TwoWay}"
AddContactVisibility="Collapsed"
Margin="0"
Grid.Column="1"/>
</Grid>
<TextBlock Text="Номер вводится без 8-ки. Пример: (XXX)XXX-XX-XX"
FontSize="15"
Foreground="Gray"
TextWrapping="WrapWholeWords"/>
</StackPanel>
</controls:HeaderedContentControl>
<Page.BottomAppBar>
<CommandBar>
<AppBarButton Label="Далее"
Icon="Accept"
Command="{Binding RegisterCommand}"/>
</CommandBar>
</Page.BottomAppBar>