The document default style properties are not cloned on document.Clone().
There is a workaround by using the following code:

var cloning = this.document.Clone();
ClearPropertiesWithoutValueAndSetLocalValues(this.document.DefaultStyle.CharacterProperties, cloning.DefaultStyle.CharacterProperties);
ClearPropertiesWithoutValueAndSetLocalValues(this.document.DefaultStyle.ParagraphProperties, cloning.DefaultStyle.ParagraphProperties);

private void ClearPropertiesWithoutValueAndSetLocalValues(DocumentElementPropertiesBase propertiesFrom, DocumentElementPropertiesBase propertiesTo)

{
	foreach (var stylePropertyFrom in propertiesFrom.StyleProperties)
	{
		var stylePropertyTo = propertiesTo.GetStyleProperty(stylePropertyFrom.PropertyDefinition);
		stylePropertyTo.ClearValue();
		if (stylePropertyFrom.HasLocalValue)
		{
			object value = stylePropertyTo.GetLocalValueAsObject();
			stylePropertyTo.SetValueAsObject(value);
		}
	}
}