_modify in propertyWrapper doesn't work

Using Swift
1 
@propertyWrapper
struct Atomic<T> {
    private var storedValue: T
    private let lock = NSLock()
    init(wrappedValue: T) { self.storedValue = wrappedValue}
    
    var wrappedValue: T {
        get {
            lock.lock()
            defer { lock.unlock() }
            return storedValue
        }
        set {
            lock.lock()
            defer { lock.unlock() }
            self.storedValue = newValue
        }
        _modify {
            lock.lock()
            defer { lock.unlock() }
            yield &storedValue
        }
    }
}

do {
    @Atomic
    var x = [354,34,3,54,6,4]
    x.append(20)
    x.append(30)
    print(x)
}

The _modify never gets called. why? Is it a bug on swift 5.7?