When SQL Using Hint returns different values

Question

Is it normal for different values ​​to appear when using hint when SQL?

I have using sub query and exist on RDBMS.

ex :

SELECT (SELECT A FROM TB_B WHERER X.I = A.I) as AA
, A.BB 
FROM TB_A A 
WHERE EXIST ( SELECT 1 FROM TB_C WHERE C.I = A.I)
ORDER BY AA ASC

then result :

11 aaa aabb
22 bbb bbaa 

But returns wrong value from upper example query. result :

11 aaa aabb
22 aaa bbaa

The RDBMS Provider says, Using NO_UNNEST hint then returns nomal values.

as far as i know SQL Hint is enhance for resource and speed.

Was i know wrong?

PS. I'm using not widely known rdbms (like oracle mysql etc.)