HTML anchor disappears when the AMP settings are enabled in a post

[carladoria]

Steps to reproduce

1. On a simple site, go to a post and insert a Heading or a Paragraph block
2. Add an Anchor HTML tag
3. Save changes
4. Enable the AMP settings on the block to Automatically fit text to container
5. Refresh the post in the edit view or save/exit and enter the edit view again
6. On a Heading Block, the Anchor HTML tag is gone
7. On a paragraph block, the anchor HTML has broken the block and there is a Attempt Recovery. When you click this one, the Anchor HTML is also gone

What I expected

For the Anchor HTML tags to remain saved after a refresh

What happened instead

They were gone after the post was saved/refreshed when the AMP setting is activated

We'd also love to know how you found the bug:
reported in 3527590-zen

Docs & troubleshooting:
https://github.com/Automattic/wp-calypso/blob/HEAD/docs/CONTRIBUTING.md
https://github.com/Automattic/wp-calypso/blob/HEAD/docs/troubleshooting.md

Helpful tips for screenshots:
https://wordpress.com/support/make-a-screenshot/
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[formosattic]

Sounds also related to #47076