Twitter returns unauthorized for user timeline but not search

I've set up an app, and have taken both the consumer key and secret from the Application Settings page (https://apps.twitter.com/app/APP_ID/keys)

I've also generated an access token and secret for my user account on this page, though I have been through the steps of the Oauth flow, authorized the application and retrieved the same token and secret via that route too.

If I use these credentials to request from the search API (i.e. https://api.twitter.com/1.1/search/tweets.json) then I get a successful response. However, when doing the same from the user timeline endpoint, despite authorising the application, I get an HTTP401 response saying 'Not authorized.'

Here is an example of what I'm doing:

import request from 'request-promise';

const twitterOauthConfig = {
    consumer_key: process.env.TWITTER_CONSUMER_KEY,
    consumer_secret: process.env.TWITTER_CONSUMER_SECRET,
    access_token_key: 'MY USER TOKEN KEY',
    access_token_secret: 'MY USER TOKEN SECRET'
};

const getRequestOptions = (user, tag) => {
    return {
        uri: 'https://api.twitter.com/1.1/statuses/user_timeline.json',
        qs: {
            q: `user_id=${user}&screen_name=${user}&tag=${tag}`
        },
        headers: {
            Accept: '*/*',
            Connection: 'close'
        },
        json: true,
        oauth: twitterOauthConfig
    };
}

const options = getRequestOptions('example_screen_name', 'example_hash_tag');
request(options)
    .then((response) => {
        // ...
    });

Does anyone know why I might be receiving that error?

via Tom Robinson