Footnotes not saved if the Custom Fields panel is open

Description

The footnotes block doesn't save new footnotes if the Custom Fields panel is open. This happens only if footnotes already exist and we want to add more of them after the post has been saved at least once.

Ticket on trac. The issue was reproduced during Test triage.

Step-by-step reproduction instructions

1. Create a post with title and some content.
2. Add a footnotes block: add 1 or 2 footnotes to your content.
3. Save your post.
4. While still in your block editor, go to Options (top right of the screen), Preferences (very bottom of the menu), Panels, enable "Custom fields", click "Show & Reload Page".
5. Now the "Custom fields" panel is displaying your post's metas.
6. Note that the meta storing the footnotes' content is named footnotes, without the _ prefix, so it is a public meta. Its value is a JSON string.
7. Edit your post content by adding another footnote and save your post.
8. Reload the page:
   • The anchor in the post content is still there.
   • The new footnote has disappeared from the footnotes block.
   • The post meta in the "Custom fields" panel doesn't contain the new footnote either.

Screenshots, screen recording, code snippet

No response

Environment info

• WordPress 6.4-alpha.
• Firefox 117.

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