No warning when leaving post page with edited metabox-fields

[boblinthorst]

WP version: 4.4 (beta 2);
using plugins: Yoast-SEO;
WP-SEO version 3.0

When leaving a post page with the content edited, it gives a warning. When editing any of the fields added by Yoast in the metabox of that post page, leaving the page won't give a warning.

Steps to duplicate:

1. Create a new post and give it proper content.
2. Publish the post.
3. Edit the created post page.
4. Edit the snippet to your heart's content and/or add a focus keyword and/or edit the advanced meta section and/or edit social media information. (all possible editable fields in the metabox).
5. Reload the page (without having updated it). It doesn't give a warning and all the stuff added by the user is gone. Needless to say, a warning would be much more user friendly. Especially because WordPress gives a warning if the content of the post has been edited.

[CarolineGeven]

Seems slightly related to #3169