Vector indexes are allowed for MERGE tables, but do not work

CREATE TABLE t (a INT, v BLOB NOT NULL, VECTOR INDEX(v)) ENGINE=MyISAM;

CREATE TABLE tm (a INT, v BLOB NOT NULL, VECTOR INDEX(v)) ENGINE=MERGE UNION=(t);

INSERT INTO t VALUES (1,x'00000000');

UPDATE tm SET a = 2;

# Cleanup

DROP TABLE tm, t;

UPDATE tm SET a = 2;

bug.vec6-1                               [ fail ]

        Test ended at 2024-09-29 23:26:10

CURRENT_TEST: bug.vec6-1

mysqltest: At line 5: query 'UPDATE tm SET a = 2' failed: ER_KEY_NOT_FOUND (1032): Can't find record in 'tm'