When I use following Swift 4 closure-based KVO way, I got a crash on iOS 10 devices

class A: NSObject {
    @objc dynamic var value: Int = 0

    var observation: NSKeyValueObservation?

    override init() {
        super.init()

        // Crash
        self.observation = self.observe(\A.value, options: [.new], changeHandler: { (_, change) in

        })
    }
}

var a: A? = A()
a = nil

Crash info:

*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'An instance 0x of class A was deallocated while key value observers were still registered with it. Current observation info: ( Context: 0x, Property: 0x>

I googled and got it was a Swift bug: https://bugs.swift.org/browse/SR-5816, and a workaround would be adding following lines inside deinit

deinit {
    if #available(iOS 11.0, *) {

    } else {
        if let observation = self.observation {
            self.removeObserver(observation, forKeyPath: "value")
        }
    }
}

My question is

1. Why following code wouldn't crash? (Just changing observing A to observing B, which is a member of A)

   class A: NSObject {
       @objc dynamic var value: Int = 0
   
       var observation: NSKeyValueObservation?
   
       let b = B()
   
       override init() {
           super.init()
           self.observation = self.b.observe(\B.value, options: [.new], changeHandler: { (_, change) in
   
           })
       }
   }
   
   class B: NSObject {
       @objc dynamic var value: Int = 0
   }
   
   var a: A? = A()
   a = nil
   

2. In the meanwhile, if I add previous workaround to above code, I got a new crash, why?

   deinit {
       if let observation = self.observation {
           self.b.removeObserver(observation, forKeyPath: "value")
       }
   }
   

New crash:

*** Terminating app due to uncaught exception 'NSRangeException', reason: 'Cannot remove an observer for the key path "value" from because it is not registered as an observer.'