Laravel FormRequest validation rules not working as expected

In my Laravel app, I am creating a dynamic search functionality using Eloquent and I have a customer search form which looks like this:

customers.search.blade.php Contrived Example

<form method="post" action="{{ route('customers.search') }}">
    @csrf
    <div class="form-group">
        <label for="first_name" class="font-weight-bold">First Name</label>
        <div class="input-group">
            <div class="input-group-prepend">
                <select name="first_name['operator']" class="custom-select">
                    <option value="%">%</option>
                    <option value="=">=</option>
                    <option value="!=">!=</option>
                </select>
            </div>
            <input id="first_name" name="first_name['query']" type="text" class="form-control">
        </div>
    </div>
</form>

I have a SearchCustomerRequest (FormRequest) class which builds out the validation rules dynamically. I've dumped it, so you can see what the generated rules looks like:

In my CustomersController under search method, I did the following to see what the validated request array looks like:

class CustomersController extends Controller
{

    // ...

    public function search(SearchCustomerRequest $searchCustomerRequest)
    {
        dd($searchCustomerRequest->validated());
    }

    // ...

}

In order to test this, in the search form, I've selected the firstname['operator'] to % and typed the first_name['query'] to test and submitted the form.

I got the following response (i.e. empty array):

[]

So, I dumped the whole request object $searchCustomerRequest to see what was in the parameter bag and this is what I see:

As you can see, my request is valid and my rules also looks correct, yet the validation doesn't seem to be working as expected.

Any ideas what might be wrong here?