Why some closures not trigger actor isolation assertions across Swift 6 and Swift 5 modules?

Using Swift
1

Consider this situation:
both the ViewModel and ViewController are compiled under Swift 6 with full concurrency checking enabled, while URLDataFetcher is still in Swift 5 mode.
I know that Swift 6 has some edge cases when calling Swift 5 modules, but the runtime behavior here really confuses me.

My most basic question is: why don’t test 1, 2, and 3 trigger an isolation assertion crash?
Are the isolation inference rules different between trailing closures and named closures?

Also, direct call the ViewModel method never crashes no matter what—unless I add @MainActor to the completionHandler parameter of its request method.

Am I missing some obvious piece of concurrency knowledge?

class ViewModel {
  let fetcher = URLDataFetcher()
  func request(url: String, completionHanlder: @escaping (String) -> Void) {
    fetcher.request(url: url, completionHanlder: completionHanlder)
  }
}

class ViewController: UIViewController {

  let viewModel = ViewModel()
  override func viewDidLoad() {
    super.viewDidLoad()

    // test 1: not crash, completionHanlder is not run in main thread
    let completionHanlder1 = { (title: String) in
      print(title)
    }
    viewModel.request(url: "", completionHanlder: completionHanlder1)
    
    // test 2: not crash, completionHanlder is not run in main thread
    viewModel.request(url: "", completionHanlder: { @MainActor (title: String) -> Void in
      print(title)
    })

    // test3: not crash, completionHanlder is not run in main thread
    let completionHanlder3 = { (title: String) -> Void in
      print(title)
    }
    viewModel.fetcher.request(url: "", completionHanlder: completionHanlder3)

    // test4: crash,  diff isolation assert
    let completionHanlder4 = { @MainActor (title: String) -> Void in
      print(title)
    }
    viewModel.fetcher.request(url: "", completionHanlder: completionHanlder4)

    // test5: crash, diff isolation assert
    viewModel.fetcher.request(url: "") { (title: String) in
      print(title)
    }
  }
}

public class URLDataFetcher {
  public init() {}
  public func request(url: String, completionHanlder: @escaping (String) -> Void) {
    DispatchQueue.global().async {
      completionHanlder("Hello, World!")
    }
  }
}