clearing file currently opened in exit pgm

So I have a stmf file opened in my exit pgm registered to
QIBM_QP0L_SCAN_OPEN.
I want to clear that file... and then later write to it further in the exit
pgm.
I do a ftruncate64() in my exit pgm and if I do a wrklnk and look at the
bytes, it did clear it and set the bytes to 0.
Good...
But when I later write to the file later on in the exit pgm, the data I
thought I cleared returns and my new data appends to it.

Is this because the file was never closed before the new data was written
to it?
How can I ensure it is cleared and then allowed to write to in this same
exit pgm without the ability to close it before the write?

Is there some functionality with the below exit pgmoutput ds fields that
can be set to allow my clear to "stick"?

8 8 CHAR(1) Update object scan information
9 9 CHAR(1) Scan status


tia

Jay