Cannot use invoke magic method within another object

Question

I have experienced something that I think it's a bug, but I'm not sure. So I come here to ask to the people who know more about this than me, is this a PHP bug? Look at the following lines:

<h1>Tests</h1>

<?php
class Foo
  {
  public function __invoke()
    {
    return 'Called correctly';
    }
  }

class Boo
  {
  public function __construct()
    {
    $this->Foo = new Foo();
    }
  }

$Foo = new Foo();
echo $Foo();

echo "<br><br><hr><br><br>";

$Boo = new Boo();
echo $Boo->Foo();

According to the specification, since The __invoke() method is called when a script tries to call an object as a function, I should obtain this:

Called correctly

---

Called correctly

But the last echo is not executed. Instead, I get this:

PHP Fatal error: Call to undefined method Boo::Foo() in [...]/index.php on line 26

Is this a bug or an expected behaviour? If it's a bug, some help about how to submit it would be highly appreciated.

EDIT based on Jon answer to show further the ugly behaviour

class Doo
  {
  public function __construct()
    {
    // The Foo class is the same as the one defined before

    $Test = new Foo();
    echo $Test();      // Echos correctly

    echo "<br><br><hr><br><br>";

    $this->Foo = $Test;
    echo $this->Foo();  // Error. Heisenbehaviour?
    }
  }

$Boo = new Doo();

Answer 1

It is not a bug, and it is not limited to Foo being an object that defines __invoke. You would get the same error with any callable:

class Foo
{
    private $bar = 'rand';

    public function __construct()
    {
        $this->bar(); // won't work
    }  
}

You need to either write this as two lines, or use call_user_func:

// #1
$Boo = new Boo();
$Foo = $Boo->Foo;
$Foo();

// #2
$Boo = new Boo();
call_user_func($Boo->Foo);