The following class

<?php

namespace App;

use IlluminateSupportStr;
use stdClass;

class Payload
{
    private(set) readonly array $validation;
    
    private array $ids = [];


    public function __construct(private readonly stdClass $payload)
    {
        $this->parseValidation();
    }

    private function parseValidation(): void
    {
        $validations = [];

        foreach ($this->payload->validation as $validation) {

            $validations[] = [
                'id' => Str::orderedUuid()->toString(),
                'field_id' => $this->ids[$validation->field_id],
                'rule' => $validation->rule,
                'value' => $this->validationValue($validation->value),
                'message' => $validation->message,
            ];
        }

        $this->validation = $validations;
    }

    private function validationValue(mixed $value): mixed
    {
        if (is_null($value)) {
            return null;
        }

        return $this->ids[$value] ?? $value;
    }
}

when analysed returns

$ phpstan analyse app/Payload.php
 1/1 [▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓] 100%

 ------ -----------------------------------------------------------------
  Line   Payload.php
 ------ -----------------------------------------------------------------
  35     Readonly property AppPayload::$validation is already assigned.
         🪪  assign.readOnlyProperty
 ------ -----------------------------------------------------------------

It appears the issue is with the call to validationValue method, but cannot figure out what and why read only association error?