Bug report

What is the current behavior?

Using the following config works in Webpack 4 but not Webpack 5:

module.exports = {
  output: {
    library: "A",
  },
  optimization: {
    runtimeChunk: "single"
  },
};

I would expect the module to be assigned to a variable A, but in Webpack 5 A is undefined.

Webpack 4 output:

var A =
(window["webpackJsonpA"] = window["webpackJsonpA"] || []).push([["main"],{
  ...
},[["./src/index.js","runtime"]]]);

Webpack 5 output:

var A;
(self["webpackChunkA"] = self["webpackChunkA"] || []).push([["main"],{
  ...
},
0,[["./src/index.js","runtime"]]]);

If the current behavior is a bug, please provide the steps to reproduce.

Smallest reproducible bug demo: https://github.com/keeganstreet/webpack-5-library

What is the expected behavior?

The module should be assigned to A.

Other relevant information:
webpack version: 5.24.2
Node.js version: v14.4.0
Operating System: Mac OS
Additional tools: N/A