NULL returned when using RETURNING in main query in combination with a CTE containing FOR UPDATE.

Hello all,

I found a possible bug or lacking documentation regarding the statement
mentioned in the subject. When using a CTE to SELECT a row before an
update, whilst locking this row so it may not change before the update is
finished, then RETURNING the row before updating using a sub-query on the
CTE will result in NULL values.

I cannot find documentation regarding the combination of a locking CTE and
UPDATE RETURNING (Only documentation about using RETURNING inside of the
CTE).

Below is a full reproduction. The last statement returns two NULL values,
unlike the query before it. I personally expect the following row to be
returned: (100, 200, 300).

wilco=> select version();
version

--------------------------------------------------------------------------------------------------------------------
PostgreSQL 14.5 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 8.5.0
20210514 (Red Hat 8.5.0-10), 64-bit
(1 row)
wilco=> CREATE TABLE acc_balance (
acc_name VARCHAR NOT NULL,
balance BIGINT NOT NULL DEFAULT 0,

PRIMARY KEY (acc_name)
);
CREATE TABLE
wilco=> INSERT INTO acc_balance (acc_name, balance) VALUES ('Wilco', 100);
INSERT 0 1
wilco=> with before as (
select balance from acc_balance where acc_name = 'Wilco'
)
update acc_balance set balance = 200 where acc_name = 'Wilco'
returning balance - (select balance from before) as delta, (select balance
from before limit 1) as before, balance;
delta | before | balance
-------+--------+---------
100 | 100 | 200
(1 row)

UPDATE 1
wilco=> with before as (
select balance from acc_balance where acc_name = 'Wilco' FOR UPDATE
)
update acc_balance set balance = 300 where acc_name = 'Wilco'
returning balance - (select balance from before) as delta, (select balance
from before limit 1) as before, balance;
delta | before | balance
-------+--------+---------
| | 300
(1 row)

UPDATE 1

Kind regards,
Wilco Kruijer