TYPO3 Core

In TYPO3 12, you could do this:

use TYPO3\CMS\Extbase\Mvc\Controller\ActionController;

class MyCustomController extends ActionController
{
    ...
    protected function overviewAction(): ResponseInterface
    {
        ...
        return $this->moduleTemplate->renderResponse();
    }
}

This would result in finding a fluid template in Resources/Private/Templates/MyCustom/Overview.html

In TYPO3 13, auto detection of controller and action name is not working, and you MUST define a template name like this

   return $this->moduleTemplate->renderResponse('MyCustom/Overview');

I tried to find the cause, and from what i understood it is a RenderingContext problem in typo3fluid/fluid/src/View/AbstractTemplateView.php

in TYPO3 12, RenderingContext in AbstractTemplateView.php has controllerName and controllerAction set fine, and in 13 both values are set to Default