Replace the DefaultNetworkLayer in Relay with superagent instead of fetch. I've noticed that when using the Laravel DebugBar the ajax query where not catched with the DefaultNetWorkLayer, switching to superagent resolved the issue.

SuperagentNetworkLayer.js

import { DefaultNetworkLayer } from 'react-relay';
import superagent from 'superagent';
import superagentRetry from 'superagent-retry';
import superagentPromise from 'superagent-promise';
import each from 'lodash/each';

// Add plugins to superagent
superagentRetry(superagent);
superagentPromise(superagent, Promise);

class SuperagentNetworkLayer extends DefaultNetworkLayer {

    buildRequest(request) {
        const superagentRequest = superagent.post(this._uri)
            .send({
                query: request.getQueryString(),
                variables: request.getVariables(),
            })
            .set('Accept', 'application/json,*/*')
            .set('Content-Type', 'application/json');

        const headers = this._init.headers || {};
        each(headers, (value, key) => {
            superagentRequest.set(key, value);
        });

        return superagentRequest;
    }

    /**
    * Sends a POST request with optional files.
    */
    _sendMutation(request) {
        const superagentRequest = this.buildRequest(request);

        const files = request.getFiles();
        if (files) {
            each(files, (file, filename) => {
                superagentRequest.attach(filename, file);
            });
        }

        return superagentRequest.then(response => ({
            json: () => response.body,
        }));
    }

    /**
    * Sends a POST request and retries if the request fails or times out.
    */
    _sendQuery(request) {
        const superagentRequest = this.buildRequest(request);

        return superagentRequest.then(response => ({
            json: () => response.body,
        }));
    }

}

export default SuperagentNetworkLayer;