Global actor method from protocol doesn't inherit to the implementation

[carlhung]

I tested the code on Linux:

protocol Pro1 {
    func foo() async 
}

protocol Pro2: Pro1 {
    @MainActor
    func foo() async
}

struct ST: Pro2 {
    func foo() {
        print("isMainThread: \(Thread.isMainThread)")
    }
}

let s = ST()

Task.detached {
    await s.foo()
}

Task {
    await s.foo()
}

dispatchMain() 

It printed out:
isMainThread: false
isMainThread: false

I was expecting both to be true.
Did I do anything wrong?