Hi Chris,

yes obviously this is the predefined behavior. If the article has a last modified date it's used, if not the current time is used as a lastmod as a fallback. By the way this is correct and can't give any SEO issues as a consequence.
It's better to show to Google that articles get modified with current date than leave it empty.

And obviously if your articles have not a last modified date, the only thing you could do is removing completely the tag from the xml sitemap if you don't want the current date.

If you really want to keep it, i would suggest to specify a last modified date by hand in the database for all your oldest articles. I can't see any other chance.

John