Cloudfront function redirect behaves different for HTTPS request

Question

The following code is a Cloudfront function designed to redirect a specific host to a new URL using a 301 Permanent Redirect.

async function handler(event) {
    const request = event.request
    const headers = request.headers
    const host = request.headers.host.value
    const uri = request.uri
    const newurl = 'https://www.newsite.com' + uri
  
    if (host === 'www.oldsite.com') {
        console.log(host)
        console.log('redirecting to www.newsite.com')
        const response = {
            statusCode: 301,
            statusDescription: 'Moved Permanently',
            headers:
                { "location": { "value": newurl } }
            }

        return response // return the altered request
    }
    return request // allow the request to continue to the origin
}

The code works as expected, and returns a 301 Moved Permanently response when navigating to http://www.oldsite.com/

However when accessing https://www.oldsite.com/ (note HTTPS) cloudfront returns a 301 response, without the status description.

Why?

Functionality in Firefox seems the same, but I thought the difference strange, and wondered where/what/why/how this was occurring.

Both responses redirect correctly to the new HTTPS site.

HTTPS request response

HTTP request response