This is a bugfix in 7.0.6, __isset() never had been called there even if it should have been:
https://3v4l.org/J0Lkk

To check if something is isset(), you generally need to check every dimension.
Is $this->a set? No it isn't (according to __isset()). isset() thus returns false.
If it were set, then isset() checks the next dimension and calls __get() to get the next dimension.

This is necessary to avoid failures in legitimate cases like (https://3v4l.org/r8U7T):

class A {
    private $foo = ["a" => [1, 2, 3]];

    function __get($a) {
        if (isset($this->foo[$a])) {
            return $this->foo[$a];
        }
        throw new Exception("Invalid key accessed! Check with isset() first!");
    }

    function __isset($a) {
        return isset($this->foo[$a]);
    }
}

$obj = new A;
var_dump(isset($obj->a[1]));
var_dump(isset($obj->b[1])); // exception in versions before 7.0.6

See also bug #69659

Maybe it's duplicate of https://bugs.php.net/bug.php?id=72223,

It seems that you make the BC break in patch release, which can be relative big problem in migration from PHP 5.6 to PHP 7.0. It's necessary to document this in migration guide at least.