Validating receipt for iOS in-app purchase always returns error 21002

I am validating my consumable in-app purchase on the server-side.

That is, I get the receipt from the client-side via:

Code Block 
        .onChange(of: self.storeObserver.paymentStatus) { status in
            switch status {
            case .purchasing:
                print("Payment status: purchasing")
            case .failed:
                self.creatingGame = false
                print("Payment status: failed")
            case .deferred:
                print("Payment status: deferred")
            case .restored:
                print("Payment status: restored")
            case .purchased:
                if Bundle.main.appStoreReceiptURL == nil {
                    print("appStoreReceiptURL is nil")
                }
                if let appStoreReceiptURL = Bundle.main.appStoreReceiptURL,
                    FileManager.default.fileExists(atPath: appStoreReceiptURL.path) {
                    do {
                        let receiptData = try Data(contentsOf: appStoreReceiptURL, options: .alwaysMapped)
                        let receiptString = receiptData.base64EncodedString(options: [])
                        print("receiptString: \(receiptString)")
                        // Read receiptData
                        createGame(receiptString: receiptString)
                    }
                    catch { print("Couldn't read receipt data with error: " + error.localizedDescription) }
                }
                
                print("Payment status: purchased")
            default:
                print("Payment status: default")
            }
        }
    private func createGame(receiptString: String){
        let data: [String:Any?] = [
            "gameName": self.gameName,
            "receipt": receiptString
        ]
        callFunction(name: "validateReceipt", data: data){ result, err in
        }

print("receiptString: \(receiptString)") prints the following:

Code Block 
Receipt data: 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

And then the receipt string is sent to the server:

Code Block 
exports.validateReceipt = functions.https.onCall(async (data, context) => {
	if (!context.auth) {
		throw new functions.https.HttpsError('permission-denied', 'The function must be called while authenticated.');
	}
	if (!data.receipt) {
		throw new functions.https.HttpsError('permission-denied', 'receipt is required');
	}
	// Now we fetch the receipt from Apple
	let body = {
		'receipt-data': data.receipt,
		'exclude-old-transactions': true
	};
	const options = {
		method: 'post',
		body: JSON.stringify(body),
		headers: {'Content-Type': 'application/json'},
	};
	return validateReceiptData('https://buy.itunes.apple.com/verifyReceipt', options, data, context);
});
function validateReceiptData(url, options, data, context) {
	var retries = 0
	return fetch(url, options).then(result => {
		return result.json();
		}).then(data => {
			if (data.status === 21007 && retries === 0) {
				retries += 1
				// Retry with sandbox URL
				console.log("Try sandbox URL");
				return validateReceiptData('https://sandbox.itunes.apple.com/verifyReceipt', options, data, context);
			}
			console.log(`data.status: ${data.status}`); // prints status code 21002
			// Process the result
			if (data.status !== 0) {
				console.log("The status code is not 0, so the receipt is invalid"); // function returns here
				return false;
			}
			const latestReceiptInfo = data.latest_receipt_info[0];
			console.log(`Receipt data is valid: ${latestReceiptInfo}`);
			if (data.type === "join"){
				return joinGame(data, context)
			} 
			else if (data.type === "create"){
				return createGame(data, context)
			}
			return 400;
		});
}

As you can see above the code tries the production verifyReceipt endpoint, and if that fails it tries the sandbox endpoint. However it never tries the sandbox endpoint as a different error comes up the first try:

Code Block 
21002
The data in the receipt-data property was malformed or the service experienced a temporary issue. Try again.

I have no idea why this error occurs. I am testing in sandbox if that makes any difference.

Any idea why I keep getting this error?