WatchConnectivity not working on device

I have implemented a very simple iOS + WatchOS app project, where I test how one can communicate with the other. I will paste the code below, but the idea is really simple. Each app has one single screen with a button and a label. Tapping the button will send a message to the counterpart indicating the timestamp when the message was generated.

If I run the app on the simulators, everything works fine: messages are sent and received correctly on both the iPhone and the Watch. (You can find a reference gif on imgur.com/ + o1ZQTLp).

The problem occurs when I try to run the same apps on my physical devices. Session is activated successfully but messages aren't sent. If I debug the code, I even see the WCSession.isReachable value set to true.

When debugging the WatchKit app, I see errorHandler is called on the func sendMessage(), and the error states:

WatchConnectivity session on paired device is not reachable.

However, the errorHandler isn't called from the iPhone app.

Details of my devices:
iOS version: 14.0.1
WatchOS version: 7.0.1

I tested this same code before installing the 7.0.1 WatchOS and it worked without any problems, so I wonder if the update introduced some error on the WatchConnectivity framework.

Code on the iPhone app:

Code Block Swift
import UIKit
import WatchConnectivity
class ViewController: UIViewController {
    @IBOutlet weak var messageLabel: UILabel!
    fileprivate var wcSession: WCSession!
    override func viewDidLoad() {
        super.viewDidLoad()
        wcSession = WCSession.default
        wcSession.delegate = self
        wcSession.activate()
    }
    @IBAction func sendMessageAction(_ sender: Any) {
        let message = [
            "content": "Message sent from iPhone on \(Date())."
        ]
        wcSession.sendMessage(message, replyHandler: nil, errorHandler: { error in
            print("Error when sending message: \(error.localizedDescription)")
        })
    }
}
extension ViewController: WCSessionDelegate {
    func session(_ session: WCSession, activationDidCompleteWith activationState: WCSessionActivationState, error: Error?) {
        switch activationState {
        case .activated:
            print("WCSession activated successfully")
        case .inactive:
            print("Unable to activate the WCSession. Error: \(error?.localizedDescription ?? "--")")
        case .notActivated:
            print("Unexpected .notActivated state received after trying to activate the WCSession")
        @unknown default:
            print("Unexpected state received after trying to activate the WCSession")
        }
    }
    func session(_ session: WCSession, didReceiveMessage message: [String : Any]) {
        guard let content = message["content"] as? String else { return }
        DispatchQueue.main.async {
            self.messageLabel.text = content
        }
    }
    func sessionDidBecomeInactive(_ session: WCSession) {
    }
    func sessionDidDeactivate(_ session: WCSession) {
    }
}

Code on the Watch Kit app:

Code Block Swift
import WatchKit
import Foundation
import WatchConnectivity
class InterfaceController: WKInterfaceController {
    @IBOutlet weak var messageLabel: WKInterfaceLabel!
    fileprivate var wcSession: WCSession!
    override func awake(withContext context: Any?) {
        wcSession = WCSession.default
        wcSession.delegate = self
        wcSession.activate()
    }
    @IBAction func sendMessageAction() {
        let message = [
            "content": "Message sent from Watch on \(Date())."
        ]
        wcSession.sendMessage(message, replyHandler: nil, errorHandler: { error in
            print("Error when sending message: \(error.localizedDescription)")
        })
    }
}
extension InterfaceController: WCSessionDelegate {
    func session(_ session: WCSession, activationDidCompleteWith activationState: WCSessionActivationState, error: Error?) {
        switch activationState {
        case .activated:
            print("WCSession activated successfully")
        case .inactive:
            print("Unable to activate the WCSession. Error: \(error?.localizedDescription ?? "--")")
        case .notActivated:
            print("Unexpected .notActivated state received after trying to activate the WCSession")
        @unknown default:
            print("Unexpected state received after trying to activate the WCSession")
        }
    }
    func session(_ session: WCSession, didReceiveMessage message: [String: Any]) {
        guard let content = message["content"] as? String else { return }
        messageLabel.setText(content)
    }
}