A DESCRIPTION OF THE PROBLEM :
Explained in the attached test.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run the attached test with --illegal-access=warn JVM option. The option is required for debugging purpose only.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
fooA.equals(fooB) == true
ACTUAL -
fooA.equals(fooB) == false

---------- BEGIN SOURCE ----------
import java.util.Arrays;
import java.util.Objects;
import java.util.StringJoiner;

/*
Run with "--illegal-access" option.
Example: java --illegal-access=warn Test.java
*/

public class Test {

public static void main(String[] args) throws Exception {
final String fooA = "foo";

// UTF-16 delimeter and Latin-1 element
// produces Latin-1 String with coder=UTF16
final String fooB = new StringJoiner("\u2013")
.add("foo")
.toString();

// fooA and fooB are should be equal (both are "foo"),
// but due to different "coder" field, the "equals" method returns false.

System.err.println(fooA.equals(fooB)); // prints false!

debugString("fooA", fooA);
debugString("fooB", fooB);
}

private static void debugString(String which, String s) throws Exception {
var coder = String.class.getDeclaredField("coder");
coder.setAccessible(true);

var value = String.class.getDeclaredField("value");
value.setAccessible(true);

System.err.println(which + ": string=\"" + s + "\" " +
"value=" + Arrays.toString((byte[])value.get(s)) +
" coder=" + coder.get(s)
);
}

}
---------- END SOURCE ----------

FREQUENCY : always