Microsoft-ui-xaml: Custom style is not applied

Describe the bug
I have a custom control derived from DropdownButton and have style for this control. But custom style is not applied.

Steps to reproduce the bug
public sealed class CustomControl1 : DropDownButton
{
public CustomControl1()
{
this.DefaultStyleKey = typeof(CustomControl1);
}

    protected override void OnApplyTemplate()
    {
        base.OnApplyTemplate();
    }
}