PHPStanのmatch式解析がよく分からん・・？

stan.php

<?php declare(strict_types = 1);
// https://phpstan.org/r/08461790-ec36-4043-8fd4-0c11074d44ea

/**
 * @param mixed $a some value
 * @return mixed
 */
function returnMatch($a): mixed
{
	return match ($a) {
		'a' => 'a',
		'x' => 'aaaa',
		'z' => 'aaaaaaaa'
	};
}

stan結果

| Line | Error |
| --- | --- |
|9 | Match expression does not handle remaining value: mixed |

実行結果.php

<?php declare(strict_types = 1);
// https://3v4l.org/hMKio

/**
 * @param mixed $a some value
 * @return mixed
 */
function returnMatch($a): mixed
{
	return match ($a) {
		'a' => 'a',
		'x' => 'aaaa',
		'z' => 'aaaaaaaa'
	};
}

echo returnMatch('x'); // aaaa