BUG #18812: Conditional rule: inconsistent check for statement

The following bug has been logged on the website:

Bug reference:      18812
Logged by:          Boris Korzun
Email address:      drtr0jan@yandex.ru
PostgreSQL version: 17.2
Operating system:   FreeBSD 14-stable
Description:

Conditional rule checks underlying table for the types ignoring the
statement.

-----
CREATE TABLE t
(
    c varchar(10) NOT NULL
);

CREATE VIEW v AS
SELECT NULL::text AS c
FROM t;

CREATE RULE "insert" AS ON INSERT TO v
    WHERE FALSE
    DO INSTEAD
    INSERT INTO t (c)
    VALUES (new.c);

CREATE RULE "skip" AS ON INSERT TO v DO INSTEAD NOTHING;
-----

I've two rules for a view - unconditional INSTEAD (skip) and conditional
INSTEAD (always FALSE). But if I trying to insert a type mismatched data to
the view, I've got a type constraint error.

-----
INSERT INTO v (c) VALUES ('testtesttest');
-----
[22001] ERROR: value too long for type character varying(10)
-----

Why? It seems like a bug.