[Template Activation] `get_block_templates` returns edited template as `index`

[kmanijak]

Description

Since template activation (#67125) the template slug for edited templates is always index. I'm using get_block_templates to retrieve a template.

For "default" template it's correct.

Step-by-step reproduction instructions

1. Add this to your functions.php

add_filter(
	'get_block_templates',
	function ( $templates ) {
		array_map(
			function ( $template ) {
				do_action( 'qm/debug', $template->slug ); // use any method to log data
			},
			$templates
		);
		return $templates;
	},
	10,
	1
);

2. Go to some page on the frontend that uses default template (e.g. 404)
3. ->404 (or whatever you chosen)
4. Go to Editor > Templates
5. Duplicate some template and make it active
6. Visit the page on the frontend
7. -> index

The same goes for title, id, etc.

The issue has significant impact on WooCommerce templates logic causing among others this problem: woocommerce/woocommerce#61032

Screenshots, screen recording, code snippet

Logged:

• slug
• id
• title

"Default" template	"Edited" template

Environment info

• WordPress 6.8
• Gutenberg 21.7RC
• WooCommerce 10.3-dev

Please confirm that you have searched existing issues in the repo.

• Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

• Yes

Please confirm which theme type you used for testing.

• Block
• Classic
• Hybrid (e.g. classic with theme.json)
• Not sure