I cannot find any more either. version 4.3.0 has some chance to be the one, if not, i cannot help.

@rik, @philip
well, I dont know the old versions, but i remember 4.3.0+ or something.
To avoid those inconsistencies, I would like to add check on java versions in future to be able to tell you from the code,
in which versions what worked quite well.

In fact, i adapted the testcases to the output of octave.

Yes, it was never documented, although really necessary.

I think the link I sent gives a hint of how to invoke it
and what it shall deliver.
Below I attached old java unit test.
I think it is easy to understand how it worked.
I had to disable that test in some version, but,
no idea when.
Note also that the test does show modifiers like static,
but it may not show inheritance information.
I am not sure how complete the old implementation really was.

    private void testFieldnamesFull(String arg) {
          this.oct.eval("f1=fieldnames(" + arg + ", '-full');");
        // ordering is immaterial
        this.oct.eval("f2={" +
                      // non-public field inaccessibleI does not show up

                      // primitive types

                      // char and Character
                      "'char cVal', 'java.lang.Character cVAL', " +
                      // boolean and Boolean
                      "'boolean zVal', 'java.lang.Boolean zVAL', " +
                      // float types
                      "'double dVal', 'java.lang.Double dVAL', " +
                      "'float fVal', 'java.lang.Float fVAL', " +
                      // integer types
                      "'long jVal', 'java.lang.Long jVAL', " +
                      "'int iVal', 'java.lang.Integer iVAL', " +
                      "'short sVal', 'java.lang.Short sVAL', " +
                      "'byte bVal', 'java.lang.Byte bVAL', " +

                      // arrays of primitive types

                      // char and Character arrays
                      "'char[] acVal', 'java.lang.Character[] acVAL', " +
                      // boolean and Boolean arrays
                      "'boolean[] azVal', 'java.lang.Boolean[] azVAL', " +
                      // float type arrays
                      "'double[] adVal', 'java.lang.Double[] adVAL', " +
                      "'float[] afVal', 'java.lang.Float[] afVAL', " +
                      // integer type arrays
                      "'long[] ajVal', 'java.lang.Long[] ajVAL', " +
                      "'int[] aiVal', 'java.lang.Integer[] aiVAL', " +
                      "'short[] asVal', 'java.lang.Short[] asVAL', " +
                      "'byte[] abVal', 'java.lang.Byte[] abVAL', " +

                      "'java.lang.Byte[][] aabVAL', " +

                      // non-primitive java.lang.Number type
                      "'java.math.BigDecimal bdVal', " +
                      // static field
                      "'static java.util.Comparator cmp', " +
                      "'java.lang.String str', " +
                      "'java.lang.String[] aStr'" +
                      "};");
        this.oct.eval("z=isempty(setxor(f1, f2));");
        assertTrue(this.oct.getBool("z"));

        // weakness: no inheritance information,
        // because the only superclass, Object, has no fields.
    }

https://uk.mathworks.com/help/matlab/ref/fieldnames.html

For fieldnames parameter '-full' disappeared.
But it is for matlab compatibility.

In old implementation, it offered modifiers type and name.
Modifiers were static final transient volatile.