Inconsistent result: "a.x <= b.y" is true in EXISTS but the equivalent JOIN returns no row (REAL vs BIGINT UNSIGNED, Range checked for each record)

CREATE TABLE a(x REAL UNIQUE NOT NULL);

CREATE TABLE b(y BIGINT UNSIGNED PRIMARY KEY);

INSERT INTO a VALUES (9223372036854775807);

INSERT INTO b VALUES (0), (9223372036854775807);

SELECT a.x, b.y, a.x <= b.y FROM a, b;

-- 9.223372036854776e18  0                    0

-- 9.223372036854776e18  9223372036854775807  1

SELECT b.y FROM b WHERE EXISTS (SELECT 1 FROM a WHERE a.x <= b.y);

-- 9223372036854775807

SELECT b.y FROM b JOIN a ON a.x <= b.y;

-- expected: 9223372036854775807   (same predicate)

-- actual:   Empty set

EXPLAIN SELECT b.y FROM b JOIN a ON a.x <= b.y;

-- a: ALL

-- b: ALL  possible_keys=PRIMARY  Extra: Range checked for each record (index map: 0x1)