I’m working on a Swift project using the Combine framework, and I’ve encountered a puzzling issue with flatMap. Here’s a simplified version of the code that demonstrates the problem:

import Combine

struct MyData {
    let value: Int
}

let publisher = PassthroughSubject<MyData, Never>()

let subscription = publisher
    .flatMap { data -> AnyPublisher<MyData, Never> in
        // Simulate a network call or some async operation
        Just(data).delay(for: .seconds(1), scheduler: RunLoop.main)
            .eraseToAnyPublisher()
    }
    .sink(receiveValue: { data in
        print("Received data: (data.value)")
    })

publisher.send(MyData(value: 1))
publisher.send(MyData(value: 2))

In the above code, I expect the sink subscriber to receive values in the order they are sent through the PassthroughSubject. However, I notice that the output sometimes appears out of order. For example, I might see:

Received data: 2
Received data: 1

This behavior seems inconsistent and I’m not sure if it’s due to how flatMap handles concurrency or if there’s another underlying issue. Could someone explain why this might be happening and how I can ensure the values are processed in the correct order?