NextScripts

Sine we been mentioned here. Bug is really in WP itself.

You can install a clean WP 5.3 without any plugins and then install and activate this simple plugin that just creates one menu item with two submenus:

<?php
/*
Plugin Name: Test Plugin
Version: 1.0.0
*/

add_action( 'admin_menu', 'testPlg_adminMenu' );   

function testPlg_adminMenu() {
    add_menu_page( 'Test Plugin', 'Test|Plugin','manage_options','testPlg','testPlg_shoPage1');
    add_submenu_page( 
      'testPlg','SubMenu 1', 
      'SubMenu 1', 'manage_options', 
      'testPlg','testPlg_shoPage1', 
      1  
    );          
    add_submenu_page( 
       'testPlg','SubMenu 1', 
       'SubMenu 1', 'manage_options', 
       'testPlg-page1', 
       'testPlg_shoPage2', 
       2  
    );          
}
?>

You will get “Warning: count(): Parameter must be an array or an object that implements Countable in /home/oradell/public_html/wp-admin/includes/plugin.php on line 1392”

Now edit the code and remove last parameters from add_submenu_page

<?php
/*
Plugin Name: Test Plugin
Version: 1.0.0
*/

add_action( 'admin_menu', 'testPlg_adminMenu' );   

function testPlg_adminMenu() {
    add_menu_page( 'Test Plugin', 'Test|Plugin','manage_options','testPlg','testPlg_shoPage1');
    add_submenu_page( 
      'testPlg','SubMenu 1', 
      'SubMenu 1', 'manage_options', 
      'testPlg','testPlg_shoPage1'
    );          
    add_submenu_page( 
       'testPlg','SubMenu 1', 
       'SubMenu 1', 'manage_options', 
       'testPlg-page1', 
       'testPlg_shoPage2'
    );          
}
?>

Everything is fine now.

According to https://developer.wordpress.org/reference/functions/add_submenu_page/ that last parameter is int and it should work.

$position
(int) (Optional) The position in the menu order this item should appear.

Default value: null

It worked ok before 5.3