Test case:

recreate table test_err (id int);
insert into test_err (ID) values (1);
insert into test_err (ID) values (2);

select *
  from test_err t
       left join (select 1 id from rdb$database) a on a.id = t.id;

Result must be
          ID ID
============ ============
           1 1
           2 <null>

But, result is
          ID ID
============ ============
           1 <null>
           2 <null>

However, the second LEFT JOIN works fine
select *
  from test_err t
       left join (select 1 id from rdb$database) a on a.id = t.id
       left join (select 2 id from rdb$database) b on b.id = t.id;

          ID ID ID
============ ============ ============
           1 <null> <null>
           2 <null> 2