Here’s my working example, which bind correctly the ViewModel and trigger correctly StringFormat and ValidateMaskDecimal:

<ListBox x:Name="Difetti"
         ItemsSource="{Binding Difetti}">
    <ListBox.ItemTemplate>
        <DataTemplate>
            <StackPanel>
                <TextBox x:Name="Metro"
                         controls:TextBoxHelper.Watermark="Metro"
                         PreviewTextInput="ValidateMaskDecimal">
                    <TextBox.Text>
                        <Binding Path="Metro"
                                 StringFormat="{}{0:0.0##########}">
                        </Binding>
                    </TextBox.Text>
                </TextBox>
            </StackPanel>
        </DataTemplate>
    </ListBox.ItemTemplate>
</ListBox>

Now, if I try to do somethings similar on code behind (removing ListBox, and bind a unique TextBox to the data model at some specific index = 0):

<TextBox x:Name="Metro"
         controls:TextBoxHelper.Watermark="Metro"
         PreviewTextInput="ValidateMaskDecimal" />
        
var data = DataContext as ViewModelData;
if (data != null && data.Difetti.Count > 0) {   
    int difettoIndex = 0;   
    Metro.SetBinding(TextBlock.TextProperty, new Binding
    {
        Source = data,
        Path = new PropertyPath($"Difetti[{difettoIndex}].Metro"),
        StringFormat = "{}{0:0.0##########}"
    });
}

it throws a ValueStringBuilder‘s ThrowFormatError exception. I don’t get why.
Where am I wrong? Shouldn’t the code give to me the same result?