Form Submits When Fields Called Directly, Fails When Using Callbacks

I’m working on a WordPress plugin and facing an issue with the Settings API. I have a settings page with two forms. The first form uses do_settings_sections() to call fields from a public function (callback). The second form renders the fields directly in the HTML.

When I submit the form that calls fields directly, it works fine. However, the form that uses do_settings_sections() and callbacks results in a “400 Bad Request” error when submitted.

Why isn’t the do_settings_sections form submitting correctly?

Here’s a simplified version of my code:

function snap_save_settings() {
    error_log('triggered snap_save_settings, Success!');
}
add_action('admin_post_snap_save_settings', 'snap_save_settings');

class Snap_Settings extends Snap_Settings_Page {
    protected function initialize_settings() {
        
        // General Settings Section
        $this->add_section('snap_main_settings', '', [$this, 'main_settings_section_callback'], 'snap-main-settings');
        $this->add_field('snap_main_settings', 'snap_linking_mode', '', [$this, 'linking_mode_callback'], 'snap-main-settings');   

    }
    
    //callback
    public function main_settings_section_callback() {
      
    }
    
    
    public function linking_mode_callback() {
        // Output a basic text input form field for testing
        echo '<label for="snap_test_input">Test Input:</label>';
        echo '<input type="text" id="snap_test_input" name="snap_test_input" value="">';
    }

    // Display Settings
    public function display_settings_page() {
        ?>
        <div class="wrap">
            <!-- Form using do_settings_sections() -->
            
            <form method="post" action="<?php echo admin_url('admin-post.php'); ?>">
                <label>This form is rendered using do_settings_sections and does not work</label>
                <input type="hidden" name="action" value="snap_save_settings">
                <?php
                settings_fields('snap_settings_group');
                do_settings_sections('snap-main-settings');
                submit_button();
                ?>
            </form>
            <br>
            <br>
            <!-- Form with fields rendered directly -->
            <form method="post" action="<?php echo admin_url('admin-post.php'); ?>">
                <label>This form is rendered directly and it works</label>
                <input type="hidden" name="action" value="snap_save_settings">
                <input type="text" name="snap_test_input" value="Test Value">
                <input type="submit" value="Save Changes">
            </form>
        </div>
        <?php
    }
}

new Snap_Settings();

Here is the settings base:

abstract class Snap_Settings_Page {
   private $sections = [];

   abstract protected function initialize_settings();

   public function __construct() {
       add_action('admin_menu', [$this, 'add_menu_page']);
       add_action('admin_init', [$this, 'register_settings']);
   }

   public function add_menu_page() {
       add_options_page(
           'Snap Settings', // Page title
           'Snap',          // Menu title
           'manage_options',     // Capability
           'snap-settings', // Menu slug
           [$this, 'display_settings_page'] // Callback function
       );
   }
   
   public function register_settings() {
       $this->initialize_settings();
   
       foreach ($this->sections as $section_id => $section) {
           add_settings_section(
               $section_id,
               $section['title'],
               $section['callback'],
               $section['page_slug']
           );
   
           foreach ($section['fields'] as $field_id => $field) {
               add_settings_field(
                   $field_id,
                   $field['title'],
                   $field['callback'],
                   $field['page_slug'],  // Use the page_slug from the field
                   $section_id
               );
               register_setting('snap_settings_group', $field_id);
           }
       }
   }

   protected function add_section($id, $title, $callback, $page_slug = 'snap-settings') {
       $this->sections[$id] = [
           'title' => $title,
           'callback' => $callback,
           'fields' => [],
           'page_slug' => $page_slug  // Store the page_slug with the section
       ];
   }

   protected function add_field($section_id, $field_id, $title, $callback, $page_slug = 'snap-settings') {
       if (!isset($this->sections[$section_id])) {
           // Optionally handle the error if the section does not exist
           return;
       }
   
       $this->sections[$section_id]['fields'][$field_id] = [
           'title' => $title,
           'callback' => $callback,
           'page_slug' => $page_slug
       ];
   }

   abstract public function display_settings_page();
   
}