COBOL WRITE after OPEN EXTEND results in CLEAR-AFTER-WRITE

Howdee!

I just debugged a strange situation, I thought I'd get the take of you
experts on it.

A COBOL400 (not ILE) program that's been working for years just showed a
deficiency, so I started debugging it.  OK, that's not too weird so far,
but it got interesting.  :)  One of the files is OPEN EXTEND within the
program, and that file started receiving blanks in some fields that we
initialized only at the beginning of the program.  So we have:

    0100-INIT.
        ...
        MOVE 'XYZ' TO SOME-FIELD
        ...

    2100-PROCESS.
        ...
        PERFORM 4200-WRITE-RECORD
        ....

    4200-WRITE-RECORD.
>>>    WRITE DETAIL-RECORD
>>>    ADD 1 TO RECS-WRITTEN



Using STRDBG ... OPMSRC(*YES), a breakpoint was set at the two lines
indicated by >>>

What I found was that just before the WRITE statement executed, everything
was cool.  SOME-FIELD had the "XYZ" in it that I expected.  Pressed F10 to
proceed to the next statement, and entered EVAL SOME-FIELD again.

I found that SOME-FIELD contained blanks.  In fact, the whole record
contained blanks.

Now maybe I'm confused, but my understanding of COBOL says that this should
not be.  I triple checked my results and found that this is consistantly
happening.  I don't know why.

We recently upgraded the OS from V4R5 to V5R1, but that was a few weeks
ago, and we've not seen this problem until today.

Anyone out here who can explain this to me?

Thanks,
Dennis