CloudFront doesn't fetch from custom origin for root url using Lambda#Edge - aws-lambda

Question

I have a Lambda#Edge function that decides which origin to use on the origin request, depending on a header value.
It doesn't work for the root url (mysite.com), but it works for subroutes (mysite.com/sth/abc). I am looking for help while trying to figure out why it doesn't work for the root url.
It looks like this:
exports.handler = (event, context, callback) => {
const request = event.Records[0].cf.request;
const isBot = headers['formaviva-agent'] && headers['formaviva-agent'][0].value === 'bot';
console.log("request before" + JSON.stringify(request));
console.log("isBot:" + isBot);
const shouldPrerender = isBot;
if (shouldPrerender) {
request.origin = {
custom: {
domainName: 'fast.formaviva.com',
port: 80,
protocol: 'http',
path: '',
sslProtocols: ['TLSv1', 'TLSv1.1'],
readTimeout: 5,
keepaliveTimeout: 5,
customHeaders: {}
}
};
request.headers['host'] = [{ key: 'host', value: 'fast.formaviva.com'}];
console.log("request after" + JSON.stringify(request));
}
callback(null, request);
};
The default origin is S3 static page hosting. formaviva-agent is set in a Viewer Request Lambda function that checks whether User-Agent belongs to a bot.
Formaviva-agent is whitelisted.
Judging by the logs, all goes well, the bot is recognized and the origin gets changed to a custom origin:
2019-05-07T09:56:45.627Z 3a2831f4-2a6b-46ea-8850-d89651a9b19e request after
{
"clientIp": "92.37.21.9",
"headers": {
"if-modified-since": [
{
"key": "If-Modified-Since",
"value": "Mon, 06 May 2019 10:50:39 GMT"
}
],
"if-none-match": [
{
"key": "If-None-Match",
"value": "W/\"41cc-16a8cc45892\""
}
],
"user-agent": [
{
"key": "User-Agent",
"value": "Amazon CloudFront"
}
],
"via": [
{
"key": "Via",
"value": "1.1 b38e161751a953866db739b688c09996.cloudfront.net (CloudFront)"
}
],
"formaviva-agent": [
{
"key": "formaviva-agent",
"value": "bot"
}
],
"x-forwarded-for": [
{
"key": "X-Forwarded-For",
"value": "92.37.21.9"
}
],
"host": [
{
"key": "host",
"value": "fast.formaviva.com"
}
]
},
"method": "GET",
"origin": {
"custom": {
"domainName": "fast.formaviva.com",
"port": 80,
"protocol": "http",
"path": "",
"sslProtocols": [
"TLSv1",
"TLSv1.1"
],
"readTimeout": 5,
"keepaliveTimeout": 5,
"customHeaders": {}
}
},
"querystring": "",
"uri": "/index.html"
}
In this case, Cloudfront still serves the content from S3 origin. The custom origin server does not get hit (confirmed). Even though the custom origin is specified. Why?