ResourceLoader queue weirdness

content_script.js

console.log( 'Start of content_script.js' );
const script = document.createElement( 'script' ),
    body = document.getElementsByTagName( 'body' )[ 0 ];
script.src = browser.extension.getURL( 'injected_script.js' );
body.appendChild( script );
console.log( 'End of content_script.js' );

injected_script.js

console.log('About to add to RLQ');
(RLQ = window.RLQ || [])
    .push( [ [ 'mediawiki.base' ], () => {
        console.log('Loaded with RL dependencies');
        mw.notify( 'This is a notification.' );
    } ] );
console.log('Finished adding to RLQ');

manifest.json

{
  "manifest_version": 2,
  "name": "WWT bug",
  "description": "",
  "author": "Sam Wilson",
  "homepage_url": "https://example.com/",
  "version": "0.1.0.0",
  "browser_specific_settings": {
    "gecko": {
      "id": "wwt-test@wikimedia"
    }
  },
  "content_scripts": [ {
    "matches": [
      "*://*.wikipedia.org/*"
    ],
    "js": [ "content_script.js" ]
  } ],
  "web_accessible_resources": [ "injected_script.js" ],
  "permissions": [ "tabs", "activeTab", "storage" ]
}