Nested Swift 4 Codable Struct uses CodingKeys irregularly

I have some JSON that I'm trying to decode with a Swift struct that conforms to the Codable protocol. The main struct doesn't seem to want to recognize the codingKey alias for thread_type, but happily consumes the explicitly named thread_id attribute. The struct in question is below:

struct Message: Codable {
var id: String?
var type: String?
var thread_id: String?
var threadType: String?
var sender: User?
var body: String?
var media: String?
var sentAt: Double?
var unread: Bool?
var status: String?
var url: String?

enum codingKeys: String, CodingKey {
    case id
    case type
    case thread_id
    case threadType = "thread_type"
    case sender
    case body
    case media
    case sentAt = "sent_at"
    case unread
    case status
    case url
}
}

The JSON that I'm trying to parse:

let json =
"""
{
"id": "Jvbl6LY",
"type": "sms",
"thread_id": "60LrVL7",
"thread_type": "578a",
"delay_until": null,
"sender": {
"id": "EVkdNBx",
"type": "user",
"first_name": "Jerry",
"last_name": "Ward",
"mobile_number": "123-456-7890",
"profile_image_url": "",
"is_online": false,
"email": "[email protected]"
},
"body": "Here is a picture of our Coquille Suite.  Let me know if you would like a reservation?",
"media": "",
"sent_at": 1509133604000.167,
"unread": false,
"status": "",
"url": "https://connect-staging.jypsee.com/api/sms/threads/60LrVL7/history/Jvbl6LY/"
}
"""

And finally the decoder call itself:

let decoder = JSONDecoder()
let data = json.data(using: .utf8)!
do {
    let message = try decoder.decode(Message.self, from: data)
    print(message.thread_id)
    print(message.threadType)
    print(message.sender?.firstName)
    print(message.sender?.lastName)
} catch {
    print(error)
}

The message.thread_id prints Optional("60LrVL7")\n" which is expected. The message.threadType prints nil\n, which is not expected. Even more bizarre is the fact that message.sender?.firstName and message.sender?.lastName both print "Optional("Jerry")\n" and "Optional("Ward")\n" respectively. Which means the nested User Codable Struct CodingKey IS working. I'm really at a loss as to why there's such an inconsistency in decoding.

Xcode Playground Gist is available here