Row Value Constructors Against Indexes Don't Work Correctly

DROP INDEX IF EXISTS TEST_INDEX ON TEST_TABLE;
DROP TABLE IF EXISTS TEST_TABLE;
CREATE TABLE IF NOT EXISTS TEST_TABLE (
    PK1 CHAR(15) NOT NULL,
    PK2 DATE NOT NULL
    CONSTRAINT PK PRIMARY KEY 
    ( 
        PK1,
        PK2 DESC
    )
);
CREATE INDEX IF NOT EXISTS TEST_INDEX ON TEST_TABLE (PK2, PK1);

UPSERT INTO TEST_TABLE (PK1, PK2) 
VALUES ('abc123',TO_DATE('2010-01-01T00:00:01Z'));
UPSERT INTO TEST_TABLE (PK1, PK2) 
VALUES ('abc123',TO_DATE('2010-01-01T00:00:02Z'));
UPSERT INTO TEST_TABLE (PK1, PK2) 
VALUES ('abc123',TO_DATE('2010-01-01T00:00:03Z'));

-- Selects 

--This select statement returns 0 rows, though it should be returning the 3 upserted rows from above
SELECT PK1, PK2
FROM TEST_TABLE 
WHERE (PK2, PK1) >= (TO_DATE('1970-01-01'), null)
-- the EXPLAIN shows the base table is being hit. Why is the base table hit with this RVC?
explain SELECT PK1, PK2
FROM TEST_TABLE 
WHERE (PK2, PK1) >= (TO_DATE('1970-01-01'), null)

--Note: by adding an ORDER BY statement, the query returns all 3 rows
SELECT PK1, PK2
FROM TEST_TABLE 
WHERE (PK2, PK1) >= (TO_DATE('1970-01-01'), null)
ORDER BY PK2, PK1;
-- the EXPLAIN shows the secondary index is now being used by this query
explain SELECT PK1, PK2
FROM TEST_TABLE 
WHERE (PK2, PK1) >= (TO_DATE('1970-01-01'), null)
ORDER BY PK2, PK1;