Random Connection Error in Google Cloud Functions (Python)

Because I have been getting some unexpected CORS errors in my browser console for the past few weeks, I have set up a super simple Python script with a single function inside Google Cloud Functions:

def prepData(request):

    if request.method == 'OPTIONS':
        headers = {
            'Access-Control-Allow-Origin': '*',
            'Access-Control-Allow-Methods': 'POST',
            'Access-Control-Allow-Headers': 'Content-Type',
            'Access-Control-Max-Age': '3600'
        }
        return ('', 204, headers)

    headers = {
        'Access-Control-Allow-Origin': '*'
    }
    return ("", 200, headers)

It looks very similar to the function presented by Google on their documentation page with one major difference: I am using POST rather than GET.

When calling the function, I am seeing a success message (Function execution took 13 ms, finished with status code: 200) in around 50% of cases and a connection error (Function execution took 11 ms, finished with status: 'connection error') in all other cases. Whenever I am getting a connection error inside GCF, Chrome's console logs an error: No 'Access-Control-Allow-Origin' header is present on the requested resource.

I am currently considering three possible causes:

1. GCP Infrastructure. I have found some related questions where Google was acknowledging some infrastructure issues.
2. Python error
3. CORS error (user sideshowbarker says: very unlikely)

What is the cause of the issue and how do I fix it?

Here is the JS code that calls the Python script:

    const xhr = new XMLHttpRequest();
    xhr.open("POST", "[URL]");
    xhr.setRequestHeader("Content-Type", "application/json");
    xhr.send(stringified_serialized_data);

Edit on 4/2/20:

I have discovered a way to not trigger the error: Add some code between the two return statements. I have not yet figured out what kind of code is needed, but d = request.get_json() appears to be sufficient. The new code looks as follows:

def prepData(request):

    if request.method == 'OPTIONS':
        headers = {
            'Access-Control-Allow-Origin': '*',
            'Access-Control-Allow-Methods': 'POST',
            'Access-Control-Allow-Headers': 'Content-Type',
            'Access-Control-Max-Age': '3600'
        }
        return ('', 204, headers)

    d = request.get_json() #THIS LINE IS NEW

    headers = {
        'Access-Control-Allow-Origin': '*'
    }
    return ("", 200, headers)