Invisible Index Getting Accessed In Trace File 11g

I am working on tuning the performance of one of the concurrent request in our 11i ERP System having database 11.1.0.7

I had enabled oradebug trace for the request and generated tkprof out of it. For below query which is taking time , I found that , in the trace generated , wait event is "db file sequential read" on an PO_LINES_N10 index but in the generated tkprof , for the same below query , the full table scan for PO_LINES_ALL is happening , as that table is 600 MB in size.

Below is the query ,
===============
UPDATE PO_LINES_ALL A
SET A.VENDOR_PRODUCT_NUM = (SELECT SUPPLIER_ITEM FROM APPS.IRPO_IN_BPAUPDATE_TMP C WHERE BATCH_ID = :B1 AND PROCESSED_FLAG = 'P' AND ACTION = 'UPDATE' AND C.LINE_ID =A.PO_LINE_ID AND ROWNUM = 1 AND SUPPLIER_ITEM IS NOT NULL),
LAST_UPDATE_DATE = SYSDATE
===============

Index PO_LINES_N10 is on the column LAST_UPDATE_DATE , logically for such query , index should not have got used as that indexed column is not in select / where clause.

Also, why there is discrepancy between tkprof and trace generated for the same query .

So , I decided to INVISIBLE the index PO_LINES_N10 but still that index is getting accessed in the trace file .

I have also checked the below parameter , which is false so optimizer should not make use of invisible indexes during query execution.

SQL> show parameter invisible

NAME TYPE VALUE
----------- optimizer_use_invisible_indexes boolean FALSE