RegisterPropertyChangedCallback works only the first time - xaml

Question

This is a follow up question related to a control that needs to intercept when common control properties changes (Inherit UserControl and hooking up to basic property events).
When any base property changes (foregound, font style, size...), I need to invalidate the canvas and redraw the content.
I've ended up with calling this method inside the UserControl constructor:
RegisterPropertyChangedCallback(DependencyProperty dp, DependencyPropertyChangedCallback callback);
For example:
RegisterPropertyChangedCallback(ForegroundProperty, OnPropertyChanged);
RegisterPropertyChangedCallback(FontFamilyProperty, OnPropertyChanged);
RegisterPropertyChangedCallback(FontSizeProperty, OnPropertyChanged);
Inside OnPropertyChanged I proceed to rewire some property and invalidate the canvas in order to update the picture.
The control is inside a DataTemplate referenced by a Pivot (PivotHeaderTemplate). The default Foreground is the unselected status (semi-transparent SystemControlForegroundBaseMediumBrush) and should get the default "white" when selected (SystemControlHighlightAltBaseHighBrush).
The control Foreground property is applied by a template, and according to the live tree it's updating, but OnPropertyChanges is called only the first time.
For example, the Pivot has 2 views: the first one starts selected and is white, and the second is unselected and is "grayish".
If I change the selection, the view changes, the Foreground property changes accordingly on both controls but OnPropertyChanges is not called, and my canvas doesn't change (because the invalidation logic is inside that method).
[UPDATE: as a workaround, I used the event LayoutUpdated to check what changed.]
For a test project, create a new UWP XAML project a new MyCanvas UserControl.
MyCanvas.xaml:
<UserControl x:Class="InheritControlProperty.Controls.MyCanvas"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:InheritControlProperty.Controls"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d">
<Canvas x:Name="content" Width="50" Height="50"/>
</UserControl>
MyCanvas.xaml.cs
using System;
using Windows.UI;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Media;
namespace InheritControlProperty.Controls
{
public sealed partial class MyCanvas : UserControl
{
private Color Value
{
get { return (Foreground as SolidColorBrush)?.Color ?? Colors.Black; }
}
public MyCanvas()
{
InitializeComponent();
RegisterPropertyChangedCallback(ForegroundProperty, OnPropertyChanged);
}
private void OnPropertyChanged(DependencyObject sender, DependencyProperty dp)
{
content.Background = new SolidColorBrush(Value);
//Logic for more complex canvas management here
}
}
}
MainPage.xaml
<Page x:Class="InheritControlProperty.MainPage"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:InheritControlProperty"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:cntr="using:InheritControlProperty.Controls"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d">
<Page.Resources>
<DataTemplate x:Key="NameTemplate">
<Grid BorderBrush="Orange" BorderThickness="2">
<cntr:MyCanvas Width="100" Height="50"/>
<!-- Try using Foreground="Red" -->
</Grid>
</DataTemplate>
</Page.Resources>
<Pivot Margin="20" HeaderTemplate="{StaticResource NameTemplate}">
<PivotItem>Item 1</PivotItem>
<PivotItem>Item 2</PivotItem>
</Pivot>
</Page>