Array to string error when using dynamic search variable

<?php

$product_id = get_the_ID(); // Replace with your product ID
$product_post = get_post($product_id);
$post_title = $product_post->post_title;

if (is_array($post_title)) {
$post_title = implode(' ', $post_title); // Convert array to string
}

$post_title = trim(sanitize_text_field($post_title));

$args = array(
's' => $post_title,
'post_type' => 'post',
'posts_per_page' => 4,
'relevanssi' => true,

);

This is my current query I run relevanssi with, however I have error (image)

I made sure that the title is not an array as you can see, however when i set static word instead of variable

eg:

‘s’ => ‘keyword’

this works correctly and no error shows.

Where could be the issue? I’m missing something?

I also tried using relevassi do query function, same result