Metaboxes markup differs by post type and has duplicated id

[stokesman]

Description

This is to document a couple of issues with markup of metaboxes in the Post Editor.

1. The "Custom Fields" metabox is in different containers depending on the type of post that’s loaded
2. There is a duplicated id="postbox-container-2"

These issues don’t have any grave problematic effects that I'm aware of though the first one was presenting a styling challenge when working on #64351

Step-by-step reproduction instructions

1. With a plugin that adds a metabox activated and the preference to show custom fields on
2. Edit a post in the Post editor
3. Note that the metabox for "Custom Fields" is in a separate parent from the metabox of the plugin
4. Edit a page in the Post editor
5. Note that the metabox for "Custom Fields" shares a parent with the metabox of the plugin

Screenshots, screen recording, code snippet

Markup while editing a Post

Markup while editing a Page

Environment info

WP 6.5.2 / Gutenberg 19.0.0

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