Using the very latest versions of both the plugin and WP. Was working fine yesterday with the other plugins I have installed. Using the Twenty Twelve theme, and all I did yesterday was some work (adding output of ACF fields) on the theme's author archive page template. Now the ACF field group I had set up and working with no issues is only displaying to my admin user, and not my test author account.
The only rule I have on the field group is "post type is equal to post". I have all default fields hidden, but my test author user is seeing all of the default fields, and NONE of the ACF ones on the edit post screen.