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) 
 ;