Inappropriate semi-consistent read in snapshot isolation

/* init */ CREATE TABLE t(a INT, b INT);

/* init */ INSERT INTO t VALUES (null, 1), (1, 1);

/* s1 */ SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED;

/* s2 */ SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED;

/* s1 */ BEGIN;

/* s2 */ BEGIN;

/* s1 */ UPDATE t SET b=3;

/* s2 */ UPDATE t SET b=2 WHERE a;  

/* s1 */ UPDATE t SET a=1;

/* s1 */ COMMIT;

/* s2 */ COMMIT;

/* s1 */ SELECT * FROM t; -- actual: [(1, 3), (1 ,2)], expected: [(1, 2), (1, 2)]