Get Electron production window to load a route with loadURL?

I've managed to build my react/electron app and have it run locally. However, the 'default' route for my app is /app, so when my app runs locally, nothing shows up. This is what I have:

public/main.js:

function createWindow() {
  mainWindow = new BrowserWindow({
    width: 400,
    height: 400,
    show: true,
    frame: false,
    transparent: true,
    webPreferences: {
      nodeIntegration: true,
      preload: `${__dirname}/preload.js`,
    },
  });
  const startURL = isDev
    ? "http://localhost:3000/app"
    : `file://${path.join(__dirname, "../build/index.html")}`;

  mainWindow.loadURL(startURL);

  mainWindow.once("ready-to-show", () => {
    // mainWindow.show()
    mainWindow.setSize();
  });
  mainWindow.on("closed", () => {
    mainWindow = null;
  });
}
app.on("ready", createWindow);

src/index.js:

ReactDOM.render(
  <React.StrictMode>
    <BrowserRouter>
      <Route path="/app" exact component={App} />
    </BrowserRouter>
  </React.StrictMode>,
  document.getElementById("root")
);

I'm using react-router to make this work. This does work locally, just not in production. To test this, I changed path="/app" to path="/" and removed exact, built it, and it worked as expected. However, I do want this to point at that particular /app route, since I want other unrelated windows to be at other endpoints. How do I get the build to recognise this properly? I've tried changing homepage: "./" to homepage: "./app" in package.json but that didn't change anything.

EDIT: I did try changing the startURL to file://${path.join(__dirname, "../build/index.html#/app")} (adding the #/app, as suggested by this answer) but that didn't work either...