PostgreSQL: Matching integer ids in where clause (apparently) fails

I do not understand the behaviour of the following query:

SELECT qs.id_gen::int                       as qs_i
     , fi.id_gen::int                       as fi_i
     , ABS(qs.id_gen::int - fi.id_gen::int) as delta
FROM peap_000.v_tlr_quantita_std qs
   , peap_000.dbg_gestore        fi
WHERE (
    ABS(qs.id_gen::int - fi.id_gen::int)   = 0
AND qs.id_tipofonte                        = fi.id_tipofonte
AND qs.id_questionario                     = fi.id_questionario
   )

• as it is, it returns 1 record (the same I would get with 'qs.id_gen::int = fi.id_gen::int' which is not what I expect from my tables);
• if I replace '= 0' with '< 0' it does not return anything (fine with me);
• if I replace '= 0' with '<= 0' it returns many records (actually, the result I do expect).

Both tables are actually views (v_... is a materialized view), id_gen is a numeric field (int4 in v_... and should also be int4 in dbg... which reports the field from a MySQL table mapped via a foreign data wrapper).

I'm quite puzzled ... What am I missing?

I add additional information and present an alternate form of the query. The following are the DDLs for the relevant part of the foreign table used:

CREATE FOREIGN TABLE peap_000.tlr_tblfontequestionario (
"IdTipoFonte" int4 NOT NULL,     -- MySQL original int(11)
"IdQuestionario" int4 NOT NULL,  -- MySQL original int(11)
"IdGen" int4 NOT NULL,...        -- MySQL original int(11)
 )
CREATE FOREIGN TABLE peap_000.tlr_tblquestionario (
"IdQuestionario" int4 NOT NULL,    -- MySQL original int(11)
"Gestore" varchar(50) NOT NULL,...
 )

Now I create a temporary table so that I can inspect the types of the colunms used by the query under examination:

CREATE  TABLE peap_000.tmp123 as
SELECT qs.id_questionario -- reported as int4 by pg_catalog inspection
 , qs.anno            -- reported as int4 by pg_catalog inspection
 , qs.id_tipofonte    -- reported as int4 by pg_catalog inspection
 , qs.id_gen          -- reported as int4 by pg_catalog inspection
 , fi.id_gen_fi
 , fi.gestore
 FROM peap_000.v_tlr_quantita_std qs -- a materialized view
 , (
 SELECT Q."IdQuestionario" AS id_questionario
          , E."Gestore"        AS gestore
      , Q."IdTipoFonte"    AS id_tipofonte
      , Q."IdGen"          AS id_gen_fi
 FROM peap_000.tlr_tblfontequestionario Q
        , peap_000.tlr_tblquestionario E
      WHERE E."IdQuestionario" = Q."IdQuestionario"
   ) fi
WHERE (qs.id_tipofonte    = fi.id_tipofonte
   AND qs.id_questionario = fi.id_questionario
   )

-- the corresponding DDL
CREATE TABLE peap_000.tmp123 (
id_questionario int4 NULL,
gestore         varchar(50) NULL,
anno            int4 NULL,
id_tipofonte    int4 NULL,
id_gen          int4 NULL,
id_gen_fi       int4 NULL
);

Now I test my simple query using the temporary table:

-- returns 740 rows: what I expect
SELECT *
  FROM peap_000.tmp123
WHERE (id_gen = id_gen_fi)

Now I try to get the same result without the tmp table but using what I think is the 'same' query (but I am evidently wrong):

-- returns 1 row: I (wrongly, I suppose) think 
-- this to be equivalent to previous query using  tmp123 table
SELECT qs.id_questionario -- reported as int4 by pg_catalog inspection
 , qs.anno            -- reported as int4 by pg_catalog inspection
 , qs.id_tipofonte    -- reported as int4 by pg_catalog inspection
 , qs.id_gen          -- reported as int4 by pg_catalog inspection
 , fi.id_gen_fi
 , fi.gestore
 FROM peap_000.v_tlr_quantita_std qs -- a materialized view
 , (
     SELECT Q."IdQuestionario" AS id_questionario
          , E."Gestore"        AS gestore
          , Q."IdTipoFonte"    AS id_tipofonte
          , Q."IdGen"          AS id_gen_fi
     FROM peap_000.tlr_tblfontequestionario Q
        , peap_000.tlr_tblquestionario E
     WHERE E."IdQuestionario" = Q."IdQuestionario"
   ) fi
WHERE (id_gen             = id_gen_fi
   AND qs.id_tipofonte    = fi.id_tipofonte
   AND qs.id_questionario = fi.id_questionario
   )

and if instead of the simple check

id_gen = id_gen_fi

I use the different guises

abs(id_gen - id_gen_fi) <= 0 or <0 or =0

I get respectively what I expect (i.e. 740 rows), the empty set, a single row. Well, I remain puzzled and unable to find the shameful error I am committing :/