Laravel 5.5 config() not showing data - php

Question

I'am working on a old project in laravel 5.5. It going file till date. But now I am configuring Laravel Socialite, added all the provider credentials to services.php. But when redirecting to provider, system throws an error, formatRedirectUrl() must be of the type array, null given.
So I checked if the providers are set correctly. To check that I used config('services.facebook.client_id'), but it returns nothing.
Note:
I have tried clearing cache: php artisan config:clear
Caching again: php artisan config:cache
Tried config('app.name') which is returning app name as expected.
Any idea what's the reason of this unexpected behaviour?
services.php:
<?php
return [
/*
|--------------------------------------------------------------------------
| Third Party Services
|--------------------------------------------------------------------------
|
| This file is for storing the credentials for third party services such
| as Stripe, Mailgun, SparkPost and others. This file provides a sane
| default location for this type of information, allowing packages
| to have a conventional place to find your various credentials.
|
*/
'facebook' => [
'client_id' => env('FACEBOOK_CLIENT_ID'),
'client_secret' => env('FACEBOOK_CLIENT_SECRET'),
'redirect' => env('CALLBACK_URL_FACEBOOK'),
],
'google' => [
'client_id' => env('GOOGLE_CLIENT_ID'),
'client_secret' => env('GOOGLE_CLIENT_SECRET'),
'redirect' => env('CALLBACK_URL_GOOGLE'),
],
'twitter' => [
'client_id' => env('TWITTER_CLIENT_ID'),
'client_secret' => env('TWITTER_CLIENT_SECRET'),
'redirect' => env('CALLBACK_URL_TWITTER'),
],
];