Xamarin.Forms ControlTemplate binding fails after control first drawn - xaml

Question

I have a ControlTemplate to modify the appearance of a radioButton in Xamarin.Forms.
I borrowed/updated it from https://learn.microsoft.com/en-us/xamarin/xamarin-forms/user-interface/radiobutton#redefine-radiobutton-appearance
It looks great, and the appearance is perfect when the page is created.
I want the image in the radiobutton to change depending on a binding, which it does not do.
The binding property gets modified, OnPropertyChanged() gets called, but the binding is not getting triggered.
Here is the general appearance (with two of these RadioButtons side by side)
Again, the appearance and the Converter and everything work just fine, but only when the page is first drawn.
Here is the ControlTemplate
<ControlTemplate x:Key="RadioButtonTemplate">
<Frame
BackgroundColor="#30888888"
HasShadow="False"
HeightRequest="{TemplateBinding HeightRequest}"
WidthRequest="{TemplateBinding WidthRequest}"
HorizontalOptions="Start"
VerticalOptions="StartAndExpand"
Padding="0">
<VisualStateManager.VisualStateGroups>
<VisualStateGroupList>
<VisualStateGroup x:Name="CheckedStates">
<VisualState x:Name="Checked">
<VisualState.Setters>
<Setter Property="BorderColor" Value="Accent" />
<Setter TargetName="check"
Property="Opacity"
Value="1" />
<Setter TargetName="checkrim"
Property="Opacity"
Value="1" />
</VisualState.Setters>
</VisualState>
<VisualState x:Name="Unchecked">
<VisualState.Setters>
<Setter Property="BackgroundColor" Value="{DynamicResource TransparentBackgroundColour}" />
<Setter Property="BorderColor" Value="Transparent" />
<Setter TargetName="check"
Property="Opacity"
Value="0" />
<Setter TargetName="checkrim"
Property="Opacity"
Value="0" />
</VisualState.Setters>
</VisualState>
</VisualStateGroup>
</VisualStateGroupList>
</VisualStateManager.VisualStateGroups>
<Grid Margin="4"
WidthRequest="100">
<ContentPresenter Margin="6" />
<Grid WidthRequest="20"
HeightRequest="20"
Padding="0"
HorizontalOptions="Start"
VerticalOptions="Start">
<Ellipse x:Name="unckecd"
Stroke="White"
StrokeThickness="2"
Fill="Transparent"
Margin="0"
WidthRequest="20" HeightRequest="20"
HorizontalOptions="Center" VerticalOptions="Center" />
<Ellipse x:Name="checkrim"
Stroke="Accent"
StrokeThickness="2"
Fill="Transparent"
Margin="0"
WidthRequest="20" HeightRequest="20"
HorizontalOptions="Center" VerticalOptions="Center" />
<Ellipse x:Name="check"
Fill="Accent"
Margin="1,1,0,0"
WidthRequest="10" HeightRequest="10"
HorizontalOptions="Center" VerticalOptions="Center" />
</Grid>
</Grid>
</Frame>
</ControlTemplate>
And the Style of my RadioButton
<Style x:Key="FancyRadioButton" TargetType="RadioButton">
<Setter Property="ControlTemplate"
Value="{DynamicResource RadioButtonTemplate}" />
</Style>
And my RadioButton
<RadioButton Style="{StaticResource FancyRadioButton}"
Value="{x:Static gdl90Types:UdpTransmitMode.UdpUnicast}"
WidthRequest="150"
HeightRequest="103">
<RadioButton.Content>
<StackLayout VerticalOptions="Start" Spacing="0" Margin="0, -3">
<Image Source="{Binding Source=ColourPresetSetting,
ConverterParameter=unicast,
Converter={StaticResource ResourceImageFromThemeConverter}}"
Margin="22,0"
Aspect="AspectFit"
/>
<Label Text="UDP UNICAST" TextColor="{DynamicResource MediumContrastTextColour}" />
</StackLayout>
</RadioButton.Content>
All I want is for the Image in the RadioButton to notice the property ColourPresetSetting changing, to update the image.
It does it when first drawn, but no longer after that.
Am I missing something obvious?