Featured image shouldn't be published until hitting the update button

[ocean90]

How to reproduce:

• choose a post and go to the edit screen
• set a featured image

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Expected behavior:
The image isn't live until I hit the update button.

What's happened:
The image is published.

Because all other changes aren't published directly the post thumbnail shouldn't published too.

(​IRC discussion)

When you say "the image is published", you actually mean that the post thumbnail ID is updated.

If it's relevant, custom fields behave the same way. I would think editing the featured image is more like editing a custom field than dealing with yet another image/attachment of the post.

Actually, it is editing a custom field, only with a fancier interface. :)

Duplicate of #12922

#21776