I know I am replying to a very old post, but I had the same problem.
This is seen when you extract pieces of a large LOB using DBMS_LOB.SUBSTR(DB_XML_FIELD, 4000, 8001) from the database, and joining them in QV to create one field that has the full XML content.
While returning data, Oracle is trimming the leading (and trailing?) white spaces.
To overcome this, I used
Replace(DBMS_LOB.SUBSTR(DB_XML_FIELD, 4000, 8001),' ','|') as XML_Chunk_2 from the database, and then replace '|' with ' ' on QV side.
Hope this helps.