I've since discovered this only occurs when you have a custom loader in your application. The second you have a class that implements Twig\Loader\LoaderInterface, the issue occurs.

I tested a fresh symfony 4 application and required twig-bundle. Before clearing the cache, the twig directory was there. After adding cache: false to twig.yaml, clearing the cache (rm -rf var/cache/dev) and then rerunning composer run post-update-cmd, the twig directory is no longer there.

However, as soon as I created my TestLoader, the exists() method was being called during this process. I know since I added an exception to it, as it shouldn't have been called.

https://github.com/nesl247/symfony-twig-cache-warmer-test is the reproduction of this issue. If you remove src/TestLoader.php, the issue goes away.

Edit: I was able to fix my issue without disabling cache by changing the priority on the custom loader to -1. However, this bug still stands because if cache is false, then the loader should never have been called to begin with.