Consider the following DDL:

recreate table zzz_tbl (
    id integer,
    company_id integer,
    hire_date date
);
insert into zzz_tbl (id, company_id, hire_date)
values (60108712, 60121225, '01.10.2004');
commit;

set term^;
create or alter procedure zzz_ppp returns (val integer)
as begin
  val=1;
  suspend;
end^
create or alter procedure ZZZ_DBG (
    COMPANY_ID integer,
    A_MONTH_BEG date)
returns (
    PERSON_ID integer)
AS
begin
for
with
inp as(
  select
     company_id
    ,dateadd(1 month to p1)-1 p2
  from(
   select
      :company_id company_id,:a_month_beg p1
      --60121225 company_id,cast('01.09.2010' as date) p1
    from
    --rdb$database
    zzz_ppp
  )t
)

,person_nfo as
(
select n.id person_id,i.p2
--from inp i join zzz_tbl n on n.company_id = i.company_id
from zzz_tbl n left join inp i on n.company_id = i.company_id
group by n.id,p2
)

select f.person_id
from person_nfo f
join zzz_tbl nt on nt.hire_date <= f.p2
into :person_id
do suspend;
end^
set term;^

--------------------------------------------

The following call of SP zzz_dbg will bring EMPTY result: select * from zzz_dbg(60121225, '01.09.2010') - that's wrong because result must have 1 row.
If we change the CTE named 'inp' in zzz_dbg like this:
inp as(
  select
     company_id
    ,dateadd(1 month to p1)-1 p2
  from(
   select
      --:company_id company_id,:a_month_beg p1
      60121225 company_id,cast('01.09.2010' as date) p1
    from
    --rdb$database
    zzz_ppp
  )t
)

-- then result will be right.

If we change CTE 'inp' in such matter:
inp as(
  select
     company_id
    ,dateadd(1 month to p1)-1 p2
  from(
   select
      :company_id company_id,:a_month_beg p1
      --60121225 company_id,cast('01.09.2010' as date) p1
    from
    rdb$database
    --zzz_ppp
  )t
)

-- result Ok.

If we do not change CTE 'inp' and modify CTE 'person_nfo' (comment GROUP BY clause):
,person_nfo as
(
select n.id person_id,i.p2
from zzz_tbl n left join inp i on n.company_id = i.company_id
--group by n.id,p2
)

-- result Ok.

If we do not change CTEs and comment out only last join:

select f.person_id
from person_nfo f
--join zzz_tbl nt on nt.hire_date <= f.p2
into :person_id

-- result also be Ok.
Also result will be Ok if this last join will be replaced from INNER to LEFT:

select f.person_id
from person_nfo f
LEFT join zzz_tbl nt on nt.hire_date <= f.p2
into :person_id


This error was encountered in: WI-V6.3.0.26074, WI-V6.3.0.26082, LI-V6.3.0.26079.
There is no such error in FB 2.1.4.