UserDefault values are missing

Question

I want to save some values in UserDefaults. And I am using this code to save

func SaveSettings(){
    let def = UserDefaults.standard
    def.set("test", forKey: "Value1")
    def.set(myString, forKey: "Value2") //value: test1
    def.set(myInt, forKey: "Value3")    //value: 25
    def.set(myInt64, forKey: "Value4")  //value: 103254
    def.synchronize() //I've tried to remove this line
}

After saving I use this code to control if my values are saved in UserDefaults or not

 for (key, value) in UserDefaults.standard.dictionaryRepresentation() {
      print("\(key) = \(value) \n")
 }

And see this result

{
   Value1 = test
   Value2 = test1
   Value3 = 25
   Value4 = 103254
}

There isn't any problem untill now. But After I restart the app and look at the values in UserDefaults I see this result

{
   Value1 = test
   Value2 =
}

As you see Value3ad Value4 are missing. However Value2 and Value1 stays. but Value2's value is missing

Answer 1

I've found the problem.

SaveSettings() func have called when myInt and myInt64's values are nil and myString's value is empty. Somehow Value3 and Value4 are deleted here. I guess this happen becaues of their value is nil. But I am not sure this is exact reason.