BUG #15069: group by after regexp_replace

The following bug has been logged on the website:

Bug reference:      15069
Logged by:          Ilhwan Ko
Email address:      koglep@gmail.com
PostgreSQL version: 9.6.7
Operating system:   macOS 10.12
Description:

create table test_t (a varchar(10), b varchar (20), c int);
 
insert into test_t (a,b,c) values ('a', 'a', 1);
insert into test_t (a,b,c) values (' a', 'a', 2); 
insert into test_t (a,b,c) values ('    a', 'a', 4);
insert into test_t (a,b,c) values ('     a', 'a', 8);
insert into test_t (a,b,c) values ('b', 'b', 1);
insert into test_t (a,b,c) values (' b', 'b', 2); 
insert into test_t (a,b,c) values ('    b', 'b', 4);
insert into test_t (a,b,c) values ('     b', 'b', 8);
 
 
select keyword, sum(t.c) as cc
from
(
    select upper(regexp_replace(a, '\\s+', '')) as keyword,  c
    from test_t
) t
group by keyword;

select upper(regexp_replace(a, '\\s+', '')) as keyword,  sum(c)
from test_t
group by upper(regexp_replace(a, '\\s+', ''));
    
    

select keyword, sum(t.c) as cc
from
(
    select upper(regexp_replace(b, '\\s+', '')) as keyword,  c
    from test_t
) t
group by keyword;

select upper(regexp_replace(b, '\\s+', '')) as keyword,  sum(c)
from test_t
group by upper(regexp_replace(b, '\\s+', ''));
    

I expected to get the same results regarding to above  four queries.
However, they were different.