Adding HTML anchor to heading block creates a problem displaying block in app

[rachelmcr]

Describe the bug

When I add an HTML anchor to my heading block on the web, the block in the app appears with the error message "Problem displaying block."

To Reproduce
Steps to reproduce the behavior:

1. Create a new post on the web.
2. Add a heading block.
3. In the block settings, under Advanced, enter an HTML anchor.
4. Save or publish the post.
5. Open the same post in the app.

Expected behavior

I expect to see the heading block appear in the app with the same settings as on the web.

Screenshots

Web editor with HTML anchor settings:

App editor:

Visual	HTML

Smartphone (please complete the following information):

• Device: iPhone XS
• OS: iOS 13.3.1
• Version: WPiOS 14.2 app store version

[hypest]

Added the headinganchor string as anchor to the block from the web, and app logs show: 'Block validation: Expected attributes %o, instead saw %o.', [], [ [ 'id', 'headinganchor', true ] ] so, this is a block validation issue that is probably caused by not proper support/preservation of the anchor data. Let's tackle this sooner rather than later.

@ceyhun , can you have a look at this one? Thanks!