BUG #5182: query with deferents results

The following bug has been logged online:

Bug reference: 5182
Logged by: artur saldanha
Email address: artur(dot)saldanha(at)gmail(dot)com
PostgreSQL version: 8.3.5
Operating system: Fedora 64
Description: query with deferents results
Details:

PostgreSQL 8.3.5 on x86_64-redhat-linux-gnu, compiled by GCC gcc (GCC) 4.3.2
20081007 (Red Hat 4.3.2-6
this query presents 7 rows...
SELECT *
FROM ( (SELECT n.nrnota, i.cdproduto, i.qtde as batata, i.vrpunit,
n.cdfinalidade, cdnrnota
FROM nfiscais n
JOIN itnfiscais i ON n.cnnota = i.cnnota
WHERE n.cdfinalidade = '5A7'::bpchar AND n.status = '131'::bpchar)
UNION
(SELECT n.nrnotaorigem AS nrnota, i.cdproduto, i.qtde *
(-1)::numeric AS batata, i.vrpunit, n.cdfinalidade, cdnrnota
FROM nfiscais n
JOIN itnfiscais i ON n.cnnota = i.cnnota
WHERE n.cdfinalidade = '5A8'::bpchar AND n.status = '131'::bpchar)) q
where nrnota = '009426'

same query that omit a colum cdnrnota present only 3 rows..
the rows omitted are in second query and rows omitted are equal contents.