NotificationService is not called

I have read through many forums but haven't found a solution for myself. When sending a notification, my NotificationService is not being called. I added it exactly as described in the official Apple tutorial and tried adding it again following other tutorials. Nothing helped. I haven't made any changes to it; it is in the same form as it is automatically created. Here it is:

import UserNotifications

class NotificationService: UNNotificationServiceExtension {

    var contentHandler: ((UNNotificationContent) -> Void)?
    var bestAttemptContent: UNMutableNotificationContent?

    override func didReceive(_ request: UNNotificationRequest, withContentHandler contentHandler: @escaping (UNNotificationContent) -> Void) {
        self.contentHandler = contentHandler
        bestAttemptContent = (request.content.mutableCopy() as? UNMutableNotificationContent)
        
        if let bestAttemptContent = bestAttemptContent {
            // Modify the notification content here...
            bestAttemptContent.title = "\(bestAttemptContent.title) [modified]"
            
            contentHandler(bestAttemptContent)
        }
    }
    
    override func serviceExtensionTimeWillExpire() {
        // Called just before the extension will be terminated by the system.
        // Use this as an opportunity to deliver your "best attempt" at modified content, otherwise the original push payload will be used.
        if let contentHandler = contentHandler, let bestAttemptContent =  bestAttemptContent {
            contentHandler(bestAttemptContent)
        }
    }
}

It does not print anything or hit any breakpoints. The notification payload looks like this:

{
    "Simulator Target Bundle": "my.bundle",
    "aps": {
        "alert": {
            "title": "Its title",
            "body": "Very nice text"
        },
        "sound": "default",
        "category": "CustomSamplePush",
        "mutable-content": 1
    }
}

Things I've tried:

• Ensured the minimum version matches that of the application.
• Configured info.plist, including UNNotificationExtensionCategory.
• Ensured "Copy Only When Installed" is unchecked.

The Content extension works. However, prints and breakpoints are also not working.