declare(strict_types=1);

/** @var \stdClass[] $items */
$items = [];

foreach ($items as $item) {
    foreach ($item->subItems as $subItem) {
        echo $subItem->_works;   // works
        echo $subItem->willFail; // works

        if (isset($subItem->someOtherAttribute)) {
            // also fails with explicit type-hint to mixed, object and \stdClass
            echo $subItem->willFail; // fails, worked before
        }
    }
}