BUG #15974: Concact with || doesn't work, but function CONCAT () works

The following bug has been logged on the website:

Bug reference: 15974
Logged by: Tessari Andrea
Email address: tessari@gmail.com
PostgreSQL version: 11.4
Operating system: linux - Red Hat 7.6
Description:

Example.
( I test on postgres 11.4 and postgres 10.9 (edb) )

create table AA3 (CLIV CHAR(2), SSTT CHAR(1));
INSERT INTO AA3 (CLIV,SSTT) VALUES (' ','2');
COMMIT;
---errore expect "A Z" --> result "AZ"
select 'A'||(CASE WHEN J01.CLIV = ' ' THEN ' '
ELSE J01.CLIV END)||'Z' as X
from AA3 J01;

---errore expect "A "--> result "A"
select 'A'||(CASE WHEN J01.CLIV = ' ' THEN ' '
ELSE J01.CLIV
END) as X
from AA3 J01;

---errore expect " Z"--> result "Z"
select (CASE WHEN J01.CLIV = ' ' THEN ' '
ELSE J01.CLIV
END)||'Z' as X
from AA3 J01;

--WORKS - show " "--> result " "
select
(CASE WHEN J01.CLIV = ' ' THEN ' '
ELSE J01.CLIV
END) as X
from AA3 J01;

-- WORKS
select 'A'||(CASE WHEN CLIV = ' ' THEN ' ' ELSE ' ' END)||'Z' from AA3
;

-- WORKS
select CONCAT ('A', x,'Z') FROM
(SELECT CASE WHEN CLIV = ' ' THEN ' ' ELSE CLIV END AS X from AA3)
;