Static properties in traits don't override static properties from parent class

[vscing]

Description

The following code:

<?php

class A
{
    protected static $instance = null;

    public static function getInstance(...$args)
    {
        if(!isset(self::$instance)){
            self::$instance = new static(...$args);
        }
        return self::$instance;
    }

    public function getB()
    {
        return B::class::getInstance();
    }
}

class B extends A
{
    protected static $instance = null;

    public static function getInstance(...$args)
    {
        if(!isset(self::$instance)){
            self::$instance = new static(...$args);
        }
        return self::$instance;
    }
}
var_dump(A::getInstance()->getB()); // B

Resulted in this output:

<?php

trait Singleton
{
    protected static $instance = null;

    public static function getInstance(...$args)
    {
        if(!isset(self::$instance)){
            self::$instance = new static(...$args);
        }
        return self::$instance;
    }
}

class A
{
    use Singleton;

    public function getB()
    {
        return B::class::getInstance();
    }
}

class B extends A
{
    use Singleton;
}
var_dump(A::getInstance()->getB()); // A

But I expected this output instead:

var_dump(A::getInstance()->getB()); // B

PHP Version

PHP 8.2.3

Operating System

mac 13.0.1

[iluuu1994]

Simplified example:

https://3v4l.org/Dll1m

class A {
    static $test;
}

class B extends A {
    static $test;
}

A::$test = 'A';
B::$test = 'B';
var_dump(A::$test, B::$test);
// string(1) "A"
// string(1) "B"

https://3v4l.org/SClLn

trait Foo {
    static $test;
}

class A {
    use Foo;
}

class B extends A {
    use Foo;
}

A::$test = 'A';
B::$test = 'B';
var_dump(A::$test, B::$test);
// string(1) "B"
// string(1) "B"

The RFC is super old, there's no mention on how this is supposed to behave. The docs aren't telling either. It's reasonable to expect the same behavior as the case without traits.