Newsletter Success Message comes from this file :
If you open this file, there is function called private function getSuccessMessage which return this message.
I had the same problem with magento 2.3, where Add to Cart button refreshed the page instead of adding anything in cart. where as website was working properly in Private Browsing on a custom theme.
Solutions Didn't Worked:
max_input_time = 600
max_execution_time = 3600
Solution Worked for me:
In Configuration >> ...
Use this to solve your issue you missed private variables in class
* Class ViewAllData
* @package Ts\Catalog\Helper
class ViewAllData extends \Magento\Framework\App\Helper\AbstractHelper
* @var \Magento\Framework\App\Config\ScopeConfigInterface