Codeigniter 4 throws errors 'user_group' keyword

Question

This is how I print incoming requests to the screen.

var_dump($this->request->getVar());

Incoming Data

order: user_group

order_type: asc

Output

array(1) {
  ["/403_shtml"]=>
  string(0) ""
}

Then I'm changing data like this:

Incoming Data

order: group

order_type: asc

Output

array(7) {
  ["order"]=>
  string(5) "group"
  ["order_type"]=>
  string(3) "asc"
}

Why might this be happening?

Note: This happens every time when I'm using 'user_group' keyword.