Method always returns the same value (cache?)

Hello,

I had a big mistake today that I was not able to solve.
I have a method to generate a random string:

PHP Code:

function RS_RANDOM_STRING(int $length, bool $onlyLetters = false, bool $onlyNumbers = false): string
 {
 $chars = '';
 if (!$onlyNumbers || $onlyLetters) $chars .= 'abcdefghijklmnopqrstuvwxyz';
 if (!$onlyLetters || $onlyNumbers) $chars .= '0123456789';
 
 $numChars = strlen($chars);
 $loop = ceil($length / $numChars);
 
 return substr(str_shuffle(str_repeat($chars, $loop)), 0, $length);
 } 


Sometimes, when this method is called, it always returns the same value as if it has been cached.
Another strange thing is that if I call the following method instead of the previous one, I get a new value at each call so no more "cache" mistake.

PHP Code:

function RS_GENERATE_GUID(bool $lowercase = true, bool $separated = false): string
 {
 mt_srand((int)microtime()*10000);
 $charid = md5(uniqid(rand(), true));
 $hyphen = $separated ? chr(45) : ''; // chr(45) = "-"
 $guid = substr($charid, 0, 8).$hyphen
 .substr($charid, 8, 4).$hyphen
 .substr($charid,12, 4).$hyphen
 .substr($charid,16, 4).$hyphen
 .substr($charid,20,12);
 
 return $lowercase ? $guid : strtoupper($guid);
 } 


I'm pretty sure I didn't have this problem with older versions of the framework and when I was in PHP 7.4 (now 8.1.2).
Does anyone understand why the first method seems to be cached sometimes and always return the same value and the second one always return a new value?