`NOT IN` over a RIGHT JOIN-produced NULL row returns TRUE for one probe value

the inner `RIGHT JOIN` can only yield one `NULL` `t3.c0`, so both probes `1` and `2` should evaluate `t2.c0 NOT IN (subquery)` to `NULL`. MariaDB instead turns the second probe into `TRUE`.

CREATE TABLE t0(c0 INT PRIMARY KEY);
CREATE TABLE t2(c0 REAL PRIMARY KEY);
CREATE TABLE t3(c0 BOOLEAN PRIMARY KEY);

INSERT INTO t0 VALUES (1);
INSERT INTO t2 VALUES (1), (2);
INSERT INTO t3 VALUES (FALSE);

SELECT t2.c0 NOT IN (
SELECT t3.c0
FROM t3 RIGHT JOIN t0
ON ((t3.c0 IS NULL) AND (t3.c0 != t3.c0))
GROUP BY t3.c0
HAVING COUNT > 0
)
FROM t2
ORDER BY t2.c0; – Expected correct result: NULL ; NULL – actual Wrong result: NULL ; 1