Contact Form: Form Block doesn't retain custom Button Color Settings on remount

[fabiankaegy]

Steps to reproduce

1. Navigate to a Gutenberg Editor page and add the Form Block
2. Enter an Email Adress and Subject Line
3. Submit these
4. Click on the Submit button and change the color of the button
5. Save / Publish the Page
6. Reload the Editor page

What I expected

The expected behaviour would be, that the form would mount right back to the block view where you see all the fields you setup, including the custom color.

What happened instead

On remounting the editor displayed the initial screen again. Once you click add form again all the fields you previously setup are still there, but the submit button loses its custom color.

Browser / OS version

macOS 10.14.4
Safari 12.1
WordPress 5.1.1
Jetpack 7.2.1
Gutenberg 5.6.1

Screenshot / Video

https://youtu.be/p-723vG3Ufs

#user-report

[mrfoxtalbot]

The same happens when changing the label of the "Submit" button, It will revert to the default label.

[jeherve]

Also in 2290075-zen

[sdixon194]

Another in 2307507-zen

[jeherve]

Fixed in #13745