You will notice that when viewing the register form you will see "Passwords Match:" showing below the second confirm password box. It should be hidden initially when the page loads and only show as you start to type the second password. When you start typing your password the text will disappear.
This was introduced with: https://www.drupal.org/node/1751434
This changed the class on that wrapper field from .password-confirm-match to .password-confirm, the CSS that controls this element was not updated and still has the old id (https://github.com/drupal/drupal/blob/8.4.x/core/modules/user/css/user.m...).
This could be fixed by the (long) upcoming https://www.drupal.org/node/2293803 but is a bug that needs fixing till then.