Using StringBuffer and StringBuidler

When I run this code below, the output that comes are


But I was expecting:  blooperwhopper blooperwhoppershopper
I am confused why "whopper" is not included between blooper and shopper (second output).

public class TestClass {
  public static void main(String[] args) {

    StringBuffer s = new StringBuffer("blooper");
    StringBuilder sb = new StringBuilder(s);
    s.append("whopper");
    sb.append("shopper");
	
    System.out.println(s);
    System.out.println(sb);
  }
}

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Could you please provide me clear and detail explanation why "whopper" is not included in the second output?