InertiaJS unit test not returning validation errors

I have a similar set up to the post here: https://laracasts.com/discuss/channels/inertia/testing-inertia-with-validation

In that I'm simply trying to test validation errors with Inertia and Laravel. My controller method looks like so;

public function update(UpdateDetailsRequest $request): RedirectResponse
{
    $data = $request->validated();

    return Redirect::route('my_details.show')->with('message', 'Details successfully updated.');
}

And my Form Request

class UpdateDetailsRequest extends FormRequest
{
    public function rules(): array
    {
        return [
            'first_name'  => ['required', 'string'],
            'last_name'   => ['required', 'string'],
            // ...
        ];
    }
}

And my pest unit test

test('test validation fails', function () {

    followingRedirects()
        ->patch(route('my_details.update'), ['last_name' => 'Doe'])
        ->assertInertia(
            function (AssertableInertia $page) {
                dd($page);

                return $page->component('MyDetails')
                    ->has('errors.first_name');
            }
        );
});

I've tested using dd() above to print out the values as I get the failure Property [errors.first_name] does not exist. when running the test normally, and the dd confirms the error bag is empty.

-props: array:8 [
    "errors" => []
    //...

If I do this in the browser and inspect using Vue Dev tools I can see the error bag populating correctly when I submit my form, and every example of a test like this I've seen seems to follow this structure so I don't know why mine is not working.

I'm using Inertias useForm() helper to handle form submission on the front end.

const form = useForm({
    first_name: details.first_name,
    last_name: details.last_name,
});

const form_submit = (form, route) => {
    form.patch(route, { /**/ });
}

<form @submit.prevent="form_submit(form, route('my_details.update'))">

Anything obvious I'm missing here?