Cannot fetch internet message headers for some emails - node.js

Question

I'm using the Microsoft Graph Client for Node.js to fetch email messages. I'm only interested in the headers. The code is working, but recently I got some errors in fetching internetMessageHeaders via the API.
For some messages, the API doesn't return any data for this field (and the code wasn't checking for this, hence the errors).
I tried to manually run the queries using POSTMAN. Using ?$select=internetMessageHeaders as a query param to only fetch the headers. If I remove the query param, I can fetch the message normally (body, etc.).
I haven't been able to find anywhere in the docs why this would happen.
Any ideas on what it's happening?
Edit: Here is the node.js code I am using
const MicrosoftGraphClient = require("#microsoft/microsoft-graph-client").Client
async function fetchEmailMessageHeaders(id, credentials) {
let client = MicrosoftGraphClient.init({
authProvider: callback => callback(null, credentials.access_token)
});
let req = client.api(id).select("internetMessageHeaders");
let message = await req.get();
return message.internetMessageHeaders;
}
Sample output (value of message var):
{
"#odata.context": "https://graph.microsoft.com/v1.0/$metadata#users('156751349d3cc68b')/messages(internetMessageHeaders)/$entity",
"#odata.etag": "W/\"CQAAABYAAABcr9US8aH2RIGaOGQZwDg3AAKyfdpk\"",
"id": "AQMkADAwATM0MDAAMS1hZWIxLThjZmYALTAwAi0wMAoARgAAA3wcQrEUgfhHoZ0BD2jmyXYHAFyv1RLxofZEgZo4ZBnAODcAAAIBDAAAAFyv1RLxofZEgZo4ZBnAODcAArJaPEcAAAA=",
"internetMessageHeaders": [
{
"name": "Received",
"value": "from BY2NAM01HT225.eop-nam01.prod.protection.outlook.com (2603:10a6:803:118::39) by VI1PR0301MB2221.eurprd03.prod.outlook.com with HTTPS via VE1PR03CA0050.EURPRD03.PROD.OUTLOOK.COM; Wed, 10 Apr 2019 11:41:55 +0000"
}
...
]
}
The problem is that for some emails, the message does not contain internetMessageHeaders.
I am using an inbox subscription to receive events for new email messages from Outlook. When an event is received I use the API to retrieve the message and extract headers from it.
API reference here
Edit2:
Here is an example JSON output of an event that fails to also retrieve headers:
{
"#odata.context": "https://graph.microsoft.com/v1.0/$metadata#users('53e07bf4-bb6a-4a82-a724-37dadfb1cf11')/messages(internetMessageHeaders)/$entity",
"#odata.etag": "W/\"CQAAABQAAADdeU+v2VzbRpZMSJGral7kAAXJkA==\"",
"id": "AAMkADhmMmVmM2NjLWFhNGMtNDBlYy04NzBkLTg5MmU2OWI0ODU4MgBGAAAAAAALg6E-e6CxRLEbZfrhFaDBBwAzAAMgKF1iTp242t34rFy5AAAAAAAOAAB5l_O62bUUTqTLk6KYrl4sAAGrJcjfAAA="
}

Answer 1

It seems internetMessageHeaders are not set when an email is replied to using outlook.office365.com. I'm also not able to see message headers when inspecting the message using Outlook on Mac.