Unexpected UPDATE behavior under strict sql_mode

/* init */ CREATE TABLE t (c1 TEXT);

/* init */ INSERT INTO t VALUES ('a');

mariadb> SELECT * FROM t;

+------+

| c1   |

+------+

| a    |

+------+

1 row in set, 1 warning (0.00 sec)

mariadb> UPDATE t SET c1 = 'b' WHERE CAST(IF('a', '1', 1) AS SIGNED);

Query OK, 1 row affected, 1 warning (0.01 sec)

Rows matched: 1  Changed: 1  Warnings: 1

mariadb> SELECT * FROM t;

+------+

| c1   |

+------+

| b    |

+------+

1 row in set (0.00 sec)