Expectation for notification is fulfilled without invoking handler block

Question

In a unit test I have an expectation for notification which I provide to the method waitForExpectation: timeout:. The last one doesn't fail so the expectation is fulfilled. But the handler of the notification sometimes isn't being invoked. I know it because NSLog at the start of the handler isn't printed in this case.

XCTestExpectation *expectation = [self expectationForNotification:NotificationName
                                                           object:object
                                                          handler: ^BOOL(NSNotification *notification) {
    NSLog(@"handler");
    <...>
}];

[self waitForExpectations:@[expectation] timeout:timeout];

How is it possible that the expectation is fulfilled, but its handler hasn't been invoked?