Does the post_date match the post_modified time and does the post_date_gmt match post_modified_gmt and are the values recorded equal to the time you update the field group or are they for some time in the future. It’s definitely and odd bug. I’ve done some more digging around in code and don’t really see how this can be happening.
At this point I can tell you what I’d be doing. I’d be following the save/update code and adding thinks like:
echo $now; die;
directly into the the WP core code to see what was going on. Check out wp-includes/post.php line 3200
