@LadySolveig can you direct me on which steps to take to solve this?

I think this entire block should be removed from content plugin Joomla.php so that action log mail template is used.

 // Messaging for new items

       $default_language = ComponentHelper::getParams('com_languages')->get('administrator');
       $debug            = $this->getApplication()->get('debug_lang');

       foreach ($users as $user_id) {
           if ($user_id != $user->id) {
               // Load language for messaging
               $receiver = $this->getUserFactory()->loadUserById($user_id);
               $lang     = Language::getInstance($receiver->getParam('admin_language', $default_language), $debug);
               $lang->load('com_content');
               $message = [
                   'user_id_to' => $user_id,
                   'subject'    => $lang->_('COM_CONTENT_NEW_ARTICLE'),
                   'message'    => sprintf($lang->_('COM_CONTENT_ON_NEW_CONTENT'), $user->get('name'), $article->title),
               ];
               $model_message = $this->getApplication()->bootComponent('com_messages')->getMVCFactory()
                   ->createModel('Message', 'Administrator');
               $model_message->save($message);
           }
       }

Why would you use the actionlog template. This is nothing to do with that

please test #44243