Steps to reproduce the issue

When trying to get a module in the getAjax function I don't get the module when the page path is root or /index.php. (eg. www.mysite.com/index.php) everything works properly on any other page path (eg. www.mysite.com/some/path).

helper.php

public static function getAjax()
{
    $jinput = JFactory::getApplication()->
    $moduleTitle = $jinput->get('moduleTitle', '', 'STRING');
    $moduleId = $jinput->get('moduleId', '', 'STRING');

    // Get module twice for good measure
    $module = JModuleHelper::getModule('mod_mymodulename', $moduleTitle);
    $moduleById = JModuleHelper::getModuleById($moduleId);
	
    return var_dump($module, $moduleById);
}

Expected result

JModuleHelper::getModuleById() -> to return the module.
JModuleHelper::getModule() -> to return the module.

Actual result

JModuleHelper::getModuleById() -> returns a dummy module.
JModuleHelper::getModule() -> returns a dummy module.

System information

Database Type -> mysql
Database Version -> 8.0.22
PHP Version -> 7.4.3
Web Server -> Apache/2.4.41 (Ubuntu)
WebServer to PHP Interface -> apache2handler
Joomla! Version -> Joomla! 3.9.22 Stable [ Amani ] 6-October-2020 15:00 GMT
Joomla! Platform Version -> Joomla Platform 13.1.0 Stable [ Curiosity ] 24-Apr-2013 00:00 GMT

Additional comments

I'm working using a sample Joomla website. Also if I assign the module to all pages it will work on the "Home" page, but then the module will also appear on all other pages which is not intended.