5.8.0-beta4: rel="noopener" in WYSIWYG block element causes broken JSON in DB

Steps to reproduce:

1. Install Advanced Custom Fields PRO 5.8.0-beta3
2. Create a Gutenberg block

add_action('acf/init', 'my_acf_init');
function my_acf_init() {
	
	// check function exists
	if( function_exists('acf_register_block') ) {
		
		// register a testimonial block
		acf_register_block(array(
			'name'				=> 'testimonial',
			'title'				=> __('Testimonial'),
			'description'		=> __('A custom testimonial block.'),
			'render_callback'	=> 'my_acf_block_render_callback',
			'category'			=> 'formatting',
			'icon'				=> 'admin-comments',
			'keywords'			=> array( 'testimonial', 'quote' ),
		));
	}
}

3. Create a field group for that block, containing a WYSIWYG editor field.
4. Create a new page. Use block in page. Use a link in the WYSIWYG editor and check the external link checkbox for target=_blank. Save the page.
5. Field is stored in db (post_content) like this:

<!-- wp:acf/testimonial {"id":"block_5c8a5e69fcebb","data":{"field_5c8a5de6c98ec":"\u003cp\u003ethis block is used in acf-beta3 and here is an external \u003ca href=\u0022https://www.advancedcustomfields.com\u0022 target=\u0022_blank\u0022 rel=\u0022noopener\u0022\u003elink\u003c/a\u003e\u003c/p\u003e"},"name":"acf/testimonial","align":"","mode":"edit"} /-->

6. Update to Advanced Custom Fields PRO 5.8.0-beta4
7. Update the blocks content, save the page.
8. Field is stored in db (post_content) like this:

<!-- wp:acf/testimonial {
    "id": "block_5c8a5e69fcebb",
    "data": {
        "test_block_content": "<p>this block is used in acf-beta4 and here is an external <a href=\"https:\/\/www.advancedcustomfields.com\" target=\"_blank\" rel="\&quot;noopener\&quot; noopener noreferrer">link<\/a><\/p>",
        "_test_block_content": "field_5c8a5de6c98ec"
    },
    "name": "acf\/testimonial",
    "align": "",
    "mode": "edit"
} /-->

What's going on with the rel attribute? 😕

// Edit: Tested on WP 5.1.1

Reproduce the issue with the Advanced Custom Fields PRO beta versions, a Gutenberg block, and a WYSIWYG link using target="_blank". Compare the resulting post_content block JSON and trace how the rel attribute is serialized; done means the saved block content remains valid JSON and preserves the intended rel value.