ltijs: Deep Linking hanging when sending back the data to the LMS

Question

I'm loosing my sleep trying to make ltijs work with Deep Linking.

I've built a simple client that registers a tool against an LMS platform. I'm able to setup the tool on the platform (Moodle and Canvas) and perform launches.

The issue I've is when trying to add "Deep Linking" to my tool. Following the directions in the ltjs docs (https://cvmcosta.me/ltijs/#/deeplinking) I've setup an "lti.onDeepLinking()" handler, that redirects to an html page where some content can be selected.

I then post this form to my own tool express server, where its data gets encoded and an auto-submit form is generated and sent back to the client (and then auto-submitted to the LMS). This is where things "hang". I clearly see the POST request being sent over to the deep_link_redirect_url as requested by the original deep link request message, but the LMS then just "hangs" there until it eventually times out (I guess) and shows a generic error page...

I'm pretty sure I'm missing some vital piece of the puzzle here but I've no clue on what it could be..

const path = require("path");
const lti = require("ltijs").Provider;

const ltiKey = "myverylongandspecialltikeyfortesting";
toolUrl = "http://192.168.1.25:3010";

// setup provider
lti.setup(ltiKey, {
  url: "mongodb://127.0.0.1:3001/lticlient",
}, {
  // options
  appRoute: "/lti/launch",
  loginRoute: "/lti/login",
  cookies: {
    secure: false, // Set secure to true if the testing platform is in a different domain and https is being used
    sameSite: "None" // set it to "None" if the testing platform is in a different domain and https is being used
  },
  devMode: true, // set it to false when in production and using https,
  keysetRoute: "/lti/keys",
});

// set the lti launch callback
lti.onConnect((token, req, res) => {
  console.log("IDTOKEN", token);
  return res.send("LTI TOOL LAUNCHED!");
});

lti.onDeepLinking((token, req, res) => {
  console.log("DEEP LINKING", token);
  // Call redirect function to deep linking view
  lti.redirect(res, '/deeplink')
})

// GET request to show the selection page
lti.app.get("/deeplink", async (req, res) => {
  res.sendFile(path.join(__dirname, '/public/select.html'))
});

// POST submit from selection page with selected content item
lti.app.post("/deeplink", async (req, res) => {
  const resource = req.body
  const items = [
    {
      type: 'ltiResourceLink',
      title: resource.product,
      url: `${toolUrl}/lti/launch`,
      custom: {
        product: resource.product
      }
    }
  ]

  const form = await lti.DeepLinking.createDeepLinkingForm(res.locals.token, items, { message: 'Successfully registered resource!' })
  console.log("RETURNING SELF-SUBMITTING FORM", form);
  return res.send(form);
})

const getPlatforms = () => {
  return [
    {
      url: "http://192.168.1.239",
      name: "MoodleClient1",
      clientId: "client-id-provided-by-Moodle",
      authenticationEndpoint: "http://192.168.1.239/mod/lti/auth.php",
      accesstokenEndpoint: "http://192.168.1.239/mod/lti/token.php",
      authConfig: { method: 'JWK_SET', key: "http://192.168.1.239/mod/lti/certs.php" }
    }
  ];
}

const registerPlatforms = async () => {
  const platforms = getPlatforms();
  platforms.forEach(async (cfg) => {
    console.log(`Registering platform ${cfg.name}`);
    await lti.deletePlatform(cfg.url, cfg.clientId);
    await lti.registerPlatform(cfg);
    const platform = await lti.getPlatform(cfg.url, cfg.clientId);
    await platform.platformActive(true)
  });
}

const setup = async () => {
  await lti.deploy({ port: 3010 });
  registerPlatforms();
  console.log("platforms registered and active");
}
setup();