XML extractvalue and NULL result-Problem

Question

I've got a very strange problem: One select gets restults in my development-database, the same select in production retrieves NULL.
The following sql-construct:
SELECT (
  CASE
    WHEN personkey!=-1
    THEN
      ( SELECT extractvalue( v.xml, '/p:PersonData/p:TypedPostalAddress[@id="' ||<string> || '"]/p:PostalAddress/p:PostalCode', .....')
      FROM <tbl1>,<tbl2>
      WHERE tbl1.id=tbl2.id)
    ELSE
      ( SELECT 'no person' FROM dual )
  END ) adress,
FROM <tbl3>
WHERE <tbl3>.personkey!=-1
When I use a view containing an outer join in the ELSE-Condition of the case I'm not getting a result in production, but I do in development (same data, same oracle-version, but other instance), but when I use a view without an outer join in the else-Condition, I'm getting results also in production. The column "personkey" is not nullable.
Does anyone got an idea what could be wrong with this SQL?
Thanks in advance for help!

Answer 1

Additional Information:
When resolving the CASE to a "union all" query, I am having the following result:
query1 is working
query2 is working
joining both queries with union all => the result of the first query (with the extractvalue) is NULL, the second query is still working.