getVar() behavior on Windows and Linux

request_order string

    This directive describes the order in which PHP registers GET, POST and Cookie variables into the _REQUEST array. Registration is done from left to right, newer values override older values.

    If this directive is not set, variables_order is used for $_REQUEST contents.

https://www.php.net/manual/en/ini.core.p...uest-order

Your $_REQUEST should have GET and POST values.
This is not CodeIgniter matter, but PHP matter.

It is very wired that your $_REQUEST is an empty array.

It is a default Ubuntu 20 / Nginx / PHP-FHM 8.1 setup
No customization
I agree that it is not a CI issue, but better to warn users that getVar/REQUEST depends not only on `request_order`