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There is no issue when I test to create a new user from the registration form on your site. So let's check this again and let me know if the issue still happens.

Hello spip,

When registering a custom post type, you can enable the option Hierarchical to make the post hierarchically and set a post as a child of another post like that. Please read more here register_post_type

For connecting two posts of two post types, you can use the extension MB Relationships. (Coaching - Organisational

Regarding the post slug or URL, you also need to use a custom code to customize the slug on your own but it would need to have more complicated code. You can also read more here https://wordpress.stackexchange.com/questions/41988/redeclare-change-slug-of-a-plugins-custom-post-type

You can use this PHP code to get the post ID and create a condition to check it before removing the editor.

$post_id = null;
if ( isset( $_GET['post'] ) ) {
    $post_id = intval( $_GET['post'] );
} elseif ( isset( $_POST['post_ID'] ) ) {
    $post_id = intval( $_POST['post_ID'] );
}

the correct code will look like

// Remove the editor for a specific post. Change 'post' to your custom post type.
add_action( 'init', function () {
    $post_id = null;
    if ( isset( $_GET['post'] ) ) {
        $post_id = intval( $_GET['post'] );
    } elseif ( isset( $_POST['post_ID'] ) ) {
        $post_id = intval( $_POST['post_ID'] );
    }
    if( $post_id == 1 ) {
        remove_post_type_support( 'post', 'editor' );    
    }
} );

You can check the relationship ID and from, to side when you create the relationship. Please share some screenshots of the relationship, I will help you to find them.

Hello Jacob,

It looks like you want to add one instance of MB test block on a post/page and allow other blocks added. Please use the setting supports with multiple set to false.

'supports' => [
    // Use the block just once per post
    multiple: false,
],

Hello Jason,

Thanks for following up.

The issue has been added to the development queue and our team will work on this in a short time. I will keep you updated on this issue.

After a post is submitted successfully on the frontend, the custom field value is already saved to the custom table (if you choose to save the custom table), not waiting to approve. Then it is not possible to move the field value to the default table wp_postmeta because the field is registered to get the value from the custom table.