StringJoiner merges with itself not as expected

Consider the following code:

import java.util.StringJoiner;

public class MergeSelf {

    public static void main(String[] args) {
        final StringJoiner joiner1 = new StringJoiner(",", "[", "]").add("a").add("b");
        System.out.println("joiner1.merge(joiner1) = " + joiner1.merge(joiner1));
        System.out.println("joiner1.merge(joiner1) = " + joiner1.merge(joiner1));

        final StringJoiner joiner2 = new StringJoiner(",").add("c").add("d");
        System.out.println("joiner2.merge(joiner2) = " + joiner2.merge(joiner2));
    }
}

The output will be:

joiner1.merge(joiner1) = [a,b,a,b,]
joiner1.merge(joiner1) = [a,b,a,b,,a,b,a,b,,]
joiner2.merge(joiner2) = c,d,c,d,


The following JCK tests will fail due to this issue:

api/java_util/StringJoiner/index.html#Merge[selfSimple1]
api/java_util/StringJoiner/index.html#Merge[selfSimple1Delimiter]
api/java_util/StringJoiner/index.html#Merge[selfSimple2]
api/java_util/StringJoiner/index.html#Merge[selfSimple2Delimiter]
api/java_util/StringJoiner/index.html#Merge[selfSimpleTwice1]
api/java_util/StringJoiner/index.html#Merge[selfSimpleTwice1Delimtier]