RSRP-256929: Cannot resolve symbol '....' due to unknown DataContext

In this control template:

<ResourceDictionary
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:WpfApplication1">

<Style TargetType="{x:Type local:CustomControl1}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type local:CustomControl1}">
<Border Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}">
<Grid>
<Grid.DataContext>
<local:CustomControl1ViewModel/>
</Grid.DataContext>
<Ellipse Fill="{Binding Fill}" />
</Grid>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</ResourceDictionary>

{Binding Fill} is marked as unknown even though an instance of CustomControl1ViewModel is explicitly assigned to Grid.DataContext in the template.


namespace WpfApplication1
{
public class CustomControl1 : Control
{
static CustomControl1()
{
DefaultStyleKeyProperty.OverrideMetadata(typeof(CustomControl1), new FrameworkPropertyMetadata(typeof(CustomControl1)));
}
}

public class CustomControl1ViewModel
{
public CustomControl1ViewModel()
{
Fill = Brushes.CornflowerBlue;
}

public SolidColorBrush Fill { get; private set; }
}
}