I'm trying to auto generate the post title and slug for a custom post type upon post creation.

It's sort of a log book, the users will only write in the main content field.

The title and slug both consist of the creation date and author name.

For example:

2019-11-15 - Doe, John

This is the code:

<?php
function auto_generate_title_on_save_post( $post_id ) {

    if ( ! wp_is_post_revision( $post_id ) ) {

        // Avoid the infinite loop.
        remove_action( 'save_post', 'auto_generate_title_on_save_post' );

        if ( get_post_type( $post_id ) === 'custom_post_type' ) {  // Check and update for the specific $post_type.
            // Get current user and current date.
            $current_user       = wp_get_current_user();
            $post_creation_date = get_the_date( 'Y-m-d' );

            $my_post = array(
                'ID'         => $post_id,
                'post_name'  => $post_creation_date . ' - ' . $current_user->user_lastname . ', ' . $current_user->user_firstname,  // Construct post_slug.
                'post_title' => $post_creation_date . ' - ' . $current_user->user_lastname . ', ' . $current_user->user_firstname,  // Construct post_title.
            );
            write_log(array( 'ID' => $post_ID));
            // Update the post into the database.
            wp_update_post( $my_post, true );

            add_action( 'save_post', 'auto_generate_title_on_save_post' );
        }
    }

}
add_action( 'save_post', 'auto_generate_title_on_save_post' );
?>

Everything works fine when the custom post type has REST disabled.

When I enable 'show_in_rest' the date is missing and the following error appears:

Updating failed. Error message: The response is not a valid JSON response.

However the post and title do get updated, but as already said the date is missing:

- Doe, John

Do I have to format the date differently or use another function to make it work with the REST API?