sql_mode=ORACLE: ROWNUM is accepted in subquery, but ignored in subquery in WHERE clause

CREATE TABLE t1 (a INT);

CREATE TABLE t2 (b INT);

INSERT INTO t1 VALUES(1),(2),(3);

INSERT INTO t2 VALUES(1),(2),(3);

SET SQL_MODE=ORACLE;

SELECT a FROM t1 WHERE a IN (SELECT b FROM t2 WHERE ROWNUM < 3);

+------+

| a    |

+------+

|    1 |

|    2 |

|    3 |

+------+

3 rows in set (0.000 sec)

SELECT b FROM t2 WHERE ROWNUM < 3;

+------+

| b    |

+------+

|    1 |

|    2 |

+------+

2 rows in set (0.001 sec)

SELECT a, b FROM t1 JOIN (SELECT b FROM t2 WHERE ROWNUM < 3);

+------+------+

| a    | b    |

+------+------+

|    1 |    1 |

|    1 |    2 |

|    2 |    1 |

|    2 |    2 |

|    3 |    1 |

|    3 |    2 |

+------+------+

6 rows in set (0.001 sec)

SELECT a, b FROM t1 JOIN (SELECT b FROM t2 WHERE ROWNUM < 3) ON a = b;

+------+------+

| a    | b    |

+------+------+

|    1 |    1 |

|    2 |    2 |

+------+------+

2 rows in set (0.001 sec)