Published sns message from lambda function not received by device - node.js

Question

I have a very simple lambda function that publishes a message to a topic like so:
const AWS = require('aws-sdk');
const sns = new AWS.SNS()
const TOPIC_ARN = "arn:aws:sns:us-east-1:<topic ARN>";
const TARGET_ARN = "arn:aws:sns:us-east-1:<target ARN>";
exports.handler = async (event) => {
return sns.publish({
Message: JSON.stringify({
"default": "any value",
"GCM": "{ \"notification\": { \"body\": \"message body\", \"title\": \"message title \", \"sound\":\"default\" } , \"data\" : {\"key\" : \"value\", \"key2\" : \"value\" } }"
}),
TargetArn: TARGET_ARN,
// TopicArn: TOPIC_ARN, also tried this same result
})
.promise()
.then(() => ({statusCode: 204, body: ''}))
.catch(err => {
console.log(err);
return {statusCode: 500, body: 'sns-error'};
});
};
The function executes without issue and I can see a success event in my cloud watch logs.
The problem is that the Notification is never received by the device.
I know that the ARN is correct because I can go to the SNS console and publish the same message to the device and it is received.
I can also spin up a nodejs application on my local machine and publish the same message to the ARN and it is received, no problem.
I also configured Delivery Status Logging on the Platform Application and I can see a success response like so from the lambda function:
{
"notification": {
"messageMD5Sum": "<sting of numbers>",
"messageId": "<sting of numbers>",
"timestamp": "2021-07-23 15:35:02.167"
},
"delivery": {
"deliveryId": "<sting of text>",
"destination": "<ARN>",
"providerResponse": "{\"multicast_id\":<sting of numbers>,\"success\":1,\"failure\":0,\"canonical_ids\":0,\"results\":[{\"message_id\":\"0:<sting of text>\"}]}",
"dwellTimeMs": 28,
"attempts": 1,
"token": <correct device token>,
"statusCode": 200
},
"status": "SUCCESS"
}
Why is the lambda function the only method where the SNS notification is not being received by the device?
SOLUTION
const AWS = require('aws-sdk');
const sns = new AWS.SNS()
const TOPIC_ARN = "arn:aws:sns:us-east-1:<topic ARN>";
const TARGET_ARN = "arn:aws:sns:us-east-1:<target ARN>";
exports.handler = async (event) => {
return sns.publish({
Message: JSON.stringify({
"default": "any value",
"GCM": "{ \"notification\": { \"body\": \"message body\", \"title\": \"message title \", \"sound\":\"default\" } , \"data\" : {\"key\" : \"value\", \"key2\" : \"value\" } }"
}),
MessageStructure: 'json', //this solved the problem
TargetArn: TARGET_ARN,
})
.promise()
.then(() => ({statusCode: 204, body: ''}))
.catch(err => {
console.log(err);
return {statusCode: 500, body: 'sns-error'};
});
};
Adding MessageStructure: 'json' solved the problem.
Thanks for all suggestions.

Answer 1

Edited original question to include answer:
Adding MessageStructure: 'json' solved the problem.
Thanks for all suggestions.