SQL & PL/SQL :: Detecting Hidden Characters

I am not sure if the problem is related to hidden characters but its my best guess so far. I am trying to enhance a part of the ERD by creating a lookup for a column one of the table that uses text (finite set of values).

CREATE TABLE N_AGREEMENT_STATUS
(
STATUS_ID NUMBER(2) PRIMARY KEY,
STATUS_NAME VARCHAR2(10 BYTE)
);
INSERT ALL
INTO N_AGREEMENT_STATUS VALUES (1, 'INACTIVE')
INTO N_AGREEMENT_STATUS VALUES (2, 'ACTIVE')
INTO N_AGREEMENT_STATUS VALUES (3, 'CLOSED')
INTO N_AGREEMENT_STATUS VALUES (4, 'CANCELLED')
SELECT * FROM DUAL;

when I try to update the source table no update takes place (0 records updated) if I used the following statement:

ALTER TABLE N_AGREEMENT ADD STATUS_ID NUMBER(2);
UPDATE N_AGREEMENT SET STATUS_ID =
(
SELECT STATUS_ID
FROM N_AGREEMENT_STATUS
WHERE N_AGREEMENT.STATUS = STATUS_NAME );

but it works fine only if I used:

UPDATE N_AGREEMENT SET STATUS_ID =
(
SELECT STATUS_ID
FROM N_AGREEMENT_STATUS
WHERE N_AGREEMENT.STATUS LIKE STATUS_NAME || '%'
);

The strange thing is that when I use:

SELECT N_AGREEMENT.STATUS, N_AGREEMENT.STATUS_ID
FROM N_AGREEMENT
WHERE N_AGREEMENT.STATUS = 'ACTIVE';

it returns correct results and all status = 'ACTIVE' appear correctly!