Swift 将弱引用作为类函数引用传递时，弱引用无法按预期工作

I am already aware of the strong/weak reference concept in swift.
yet after running the next code, and tapping on the Button (and dismissing the screen), the TestViewModel stayed in memory! I was expecting that using [weak viewmodel] will be enough to prevent it. in the second example I managed to fix it - but I don't understand why it worked

import SwiftUI
import Resolver

struct TestScreen: View {
    
    @StateObject var viewmodel = TestViewModel()
    @Injected var testStruct: TestStruct
    @Environment(\.presentationMode) var presentationMode
    
    var body: some View {
        NavigationView {
            
            VStack(spacing: 0) {
                
                Button("go back") { [weak viewmodel] in
                        testStruct.saveActionGlobaly(onAsyncAction:  viewmodel?.someAsyncAction )
                        presentationMode.wrappedValue.dismiss()
                    }
            }
        }
    }
}

import Foundation
import Resolver
import SwiftUI

public class TestStruct {
   var onAsyncAction: (() async throws -> Void)?
    
    public func saveActionGlobaly(onAsyncAction: (() async throws -> Void)?) {
        self.onAsyncAction = onAsyncAction
    } 
}

EXAMPLE 2:
I managed to prevent the leak by changing the code this way: (notice the changes in the callback passed to onAsyncAction)

import Resolver

struct TestScreen: View {
    
    @StateObject var viewmodel = TestViewModel()
    @Injected var testStruct: TestStruct
    @Environment(\.presentationMode) var presentationMode
    
    var body: some View {
        NavigationView {
            
            VStack(spacing: 0) {
                
                Button("go back") { [weak viewmodel] in
                        testStruct.saveActionGlobaly(onAsyncAction:  { await viewmodel?.someAsyncAction() } )
                        presentationMode.wrappedValue.dismiss()
                    }
            }
        }
    }
}

我不明白为什么第二个TestScreen成功地应用了弱引用，而第一个没有， 谢谢(:

环境: SWIFT 5 Xcode 14.2