Perhaps there is some setting in Outlook that covers this? You might ask in
an Outlook newsgroup. Of course, the ultimate solution would be not to open
documents directly from attachments, but I can understand why people do this
(I do it myself when all I want to do is see the content, not edit it).

In any case, Outlook should not be changing your Word Options settings
(though I can't swear that it doesn't; it's certainly very arrogant about
making itself the default mail client every time it's updated), but there
are plenty of other things that could. Add-ins are the most common culprits.
I went crazy for years about an Options setting that kept changing behind my
back (luckily not a vital one, but I still found it annoying and puzzling),
and when I removed an unneeded add-in, the problem was solved. I can also
attest that every time I use Adobe Acrobat's PDFMaker, "Update fields" is
unchecked in the Print Options. So I think it would not be unreasonable to
look outside Word for an answer.