BUG #18803: ERROR: wrong varnullingrels (b) (expected (b 4)) for Var 2/1

The following bug has been logged on the website:

Bug reference: 18803
Logged by: Marko Tiikkaja
Email address: marko@joh.to
PostgreSQL version: 16.6
Operating system: Linux
Description:

Given this schema:

BEGIN;
CREATE TABLE Users();
CREATE TABLE Orders(MessageID text);
CREATE TABLE Transfers (TransferID bigint);
COMMIT;

Running this query:

DO $$
BEGIN

EXECUTE $SQL$
SELECT
CASE WHEN $1 < 1
THEN 1
ELSE sum(1) OVER ()
END AS Total,
MessageID
FROM (
SELECT
coalesce(
ECR.MessageID,
Orders.MessageID
) AS MessageID
FROM Orders
LEFT JOIN LATERAL (
SELECT
Transfers.TransferID || Orders.MessageID AS MessageID
FROM Transfers
) ECR ON TRUE
) Txn
$SQL$
USING 1;

END
$$;

produces the following error:

ERROR: wrong varnullingrels (b) (expected (b 4)) for Var 2/1

(where the "1" seems to correspond to the pg_attribute.attnum of
Orders.MessageID)