Using UNNotificationServiceExtension with FCM not getting called

Code Block 
firebaseHeaders = {
		"Content-Type":"application/json",
		"Authorization":"key=MY_KEY"
}
firebaseBody = {
		"to":device_token,
		"priority":"high",
		"mutable-content":True,
		"apns-priority":5,
		"notification": { "title_loc_key":"push.goalReachedTitle", "body_loc_key":"push.goalReachedContentRemote", "body_loc_args":[str(entry['value']), entry['region']], "sound":"default", "badge":1 },
		"data": { "values":data, "region":entry['region'], "value":entry['value'] }
}
firebaseResult = requests.post("https://fcm.googleapis.com/fcm/send", data=None, json=firebaseBody, headers=firebaseHeaders)

Code Block 
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 {
						bestAttemptContent.title = bestAttemptContent.title + " [TEST 123]"						
						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)
				}
		}
}