string validation max 15000 characters not working

I am trying to validate a textarea with max 15000 characters validation rule.

This is the rule I created:

array:1 [▼
    "text" => "required|string|max:15000"
]

Somewhere before I concat two inputs in this way:

$request->merge(['text' => $request->input('text') . PHP_EOL . PHP_EOL . $request->input('sign']);

The code does not pass the validation even if the sum of text input and sign input is (according to this website) is 14988.
To calculate it, I just past the textarea value to the website and I checked the characters count.

via Chebli Mohamed