AMP Email form causes error [amp-form] Form submission failed: Error: Request viewerRenderTemplate failed: Error: Class$obf_1008:

The form submits perfectly well in the amp playground and the amp Gmail playground. When I send myself the email and open it in Gmail, the dynamic content loads correctly but on submitting the form, I get the following errors:

Uncaught (in promise) Error: Class$obf_1008: [https://dynamicmail-pa.googleapis.com/v2/xhrs:proxy?alt=protojson] Cg: Unsupported HTTP status: 400: Class$obf_1007: [object Object]

log.js:258 [amp-form] Form submission failed: Error: Request viewerRenderTemplate failed: Error: Class$obf_1008: [https://dynamicmail-pa.googleapis.com/v2/xhrs:proxy?alt=protojson] Cg: Unsupported HTTP status: 400: Class$obf_1007: [object Object]

I understand this is likely due to CORS. On my server, I have tried using the "@ampproject/toolbox-cors" library as suggested by the official documentation as so:

const app = express();
const ampCors = require("@ampproject/toolbox-cors");

const cors = require("cors");
app.use(
    ampCors({
        email: true,
        verifyOrigin: false,
        verbose: true
    })
);

I've also tried to manually set all of the headers like so:

const whitelist = [
    "https://playground.amp.dev",
    "https://mail.google.com",
    "https://amp.gmail.dev"
];
const corsOptions = {
    origin: function (origin, callback) {
        console.log("origin", origin);
        if (!origin) {
            return callback(null, true);
        }
        if (whitelist.indexOf(origin) !== -1) {
            return callback(null, true);
        } else {
            return callback(new Error("Origin not in whitelist"));
        }
    }
};

app.use(cors(corsOptions));
app.use((req, res, next) => {
    res.set("AMP-Access-Control-Allow-Source-Origin", "amp@gmail.dev"); //I've changed this to my sender email address when testing from Gmail
    res.set(
        "Access-Control-Expose-Headers",
        "AMP-Access-Control-Allow-Source-Origin"
    );
    res.set("Access-Control-Allow-Credentials", true);
    next();
});

Both of these work fine in the playground as mentioned but the forms still cause the same error when posted from Gmail.

I've received an answer from someone at Google about this.

There's a bug with AMP forms submitting with enctype=3D"application/x-www-form-urlencoded" which has been submitted for resolution.

I'll update this thread when I have an update on the ticket.