weird laravel validation in nested array

The base problem is from client request to my API that using Content-Type: multipart/form-data header

After many hours workaround and based on explanation given by @IGP. This is the solution (probably).

reworked my FormRequest class:

<?php

namespace App\Http\Requests\Test;

use Illuminate\Foundation\Http\FormRequest;

class Test extends FormRequest
{
    public function castData() // reworked from my previous validationData()
    {
        return [
            'booleanField' => [
                "type" => "boolean",
                "default" => false,
            ],
            'nullableBooleanField' => [
                "nullable" => true,
                "type" => "boolean",
            ],
            'fileField' => [
                "type" => "file",
            ],
            'arrayField' => [
                "type" => "input",
            ],
            'arrayField.*.booleanField' => [
                "type" => "boolean",
                "default" => false,
            ],
            'arrayField.*.fileField' => [
                "type" => "file",
            ],
        ];
    }

    public function rules(): array
    {
        return [
            "booleanField" => ["required", "boolean"],
            "nullableBooleanField" => ["nullable", "boolean"],
            "fileField" => ["required", "file", "mimes:jpg,png,jpeg,docx,xlsx,zip", "max:5120"],
            "arrayField" => ["required", "array"],
            "arrayField.*.booleanField" => ["required", "boolean"],
            "arrayField.*.fileField" => ["required", "file", "mimes:jpg,png,jpeg,docx,xlsx,zip", "max:5120"],
        ];
    }

    // I created this custom function below to handle prepareForValidation
    protected function prepareForValidation(): void
    {
        if(method_exists($this, "castData")){
            $this->merge(
                $this->setDefaultToMissingData(
                    $this->resolveCasts(
                        $this->all(),
                        $this->castData()
                    ),
                    Arr::where(Arr::map($this->castData(), function($value,$key){
                        return Arr::get($value, 'default');
                    }), function($value){
                        return !is_null($value);
                    })
                )
            );
        }
    }

    private function resolveCasts(array $data, array $castData, &$discoveredDataKey = null)
    {
        return Arr::map($data, function($value, $key) use ($castData, $discoveredDataKey){
            $discoveredDataKey = ($discoveredDataKey !== null ? $discoveredDataKey.'.' : null).$key;
            if(Arr::accessible($value)){
                return $this->resolveCasts($value, $castData, $discoveredDataKey);
            }else{
                $getCast = Arr::first(Arr::where($castData, function($castValue, $castKey) use ($discoveredDataKey) {
                    return Str::replaceMatches('/\.\d+/', '.*', $discoveredDataKey) === $castKey;
                }));

                $getValue = $this->{Arr::get($getCast, "type", "input")}($discoveredDataKey, Arr::get($getCast, "default"));
                if(Arr::get($getCast, "nullable", false)){
                    $nullableValue = $this->input($discoveredDataKey);
                }
                $value = isset($nullableValue) ? (
                    is_null($nullableValue) ? null : $getValue
                ) : $getValue;

                return $value;
            }
        });
    }

    private function setDefaultToMissingData($data, $casts) {
        foreach ($casts as $cast => $value) {
            $data = $this->setDataValueToDefaultIfNotExists($data, $cast, $value);
        }
        return $data;
    }
    private function setDataValueToDefaultIfNotExists($data, $cast, $value) {
        $keys = explode('.', $cast);
        $current = &$data;

        foreach ($keys as $index => $key) {
            if ($key === '*') {
                foreach ($current as &$subData) {
                    $subData = $this->setDataValueToDefaultIfNotExists($subData, implode('.', Arr::take($keys, $index + 1)), $value);
                }
                return $data;
            }

            if ($index === count($keys) - 1) {
                if (!Arr::exists($current, $key)) {
                    $current[$key] = $value;
                }
                return $data;
            }

            if (!Arr::exists($current, $key) || !Arr::accessible($current[$key])) {
                $current[$key] = [];
            }
            $current = &$current[$key];
        }

        return $data;
    }
}

Now all working as expected.

Maybe not the best for performance. You can always improve that.

Thank you... I hope this helps someone with similar case