Restored logicals have different format level identifier to originals - what gives?

Hi folks

I've hit a problem with some logical files that have been restored to
another system. Although the field layout and attributes remain the
same, the format level ID *has* changed, and that causes CPF4131
errors :( The only difference I can see between the original file, and
the restored file, as that some of the fields show "Allows the null
value" in a DSPFFD listing of the original. The restored version
doesn't show this.

The physicals were created by SQL (they are part of a 3rd party
application) and the logicals are ones we've created (using DDS) to
provide the access to the files from our in-house systems. To try to
narrow down the problem I created a new library and used CRTDUPOBJ to
put empty copies of a couple of the physicals into it. I then used
CRTDUPOBJ to duplicate all dependant logicals to the same library,
creating them in the same order the logicals were originally compiled
in. The logicals in the new library all had matching format level
identifiers to the live system files. So far so good. Saving &
restoring this library to another system results in the logicals
without the "Allows the null value" and the mismatched level ID. :(

Can anyone explain how this situation might arise, and better still,
how I can get round it?

Regards, Martin