If phpBB Twig extension is decorated in phpBB extension, PHP fatal error is thrown, example:

Fatal error: Uncaught LogicException: Unable to register extension "rxu\templateventsort\template\twig\extension" as it is already registered. in G:\htdocs\33\vendor\twig\twig\src\ExtensionSet.php on line 149

That is because new service is an alias of the decorated core one, thus it's already registered after phpBB extension got enabled, but phpBB template engine doesn't perform the check.