WKWebView changing cookie version to 1

When I take a cookie and add it to a WKWebView's httpCookieStore, the cookie's version suddenly changes from 0 to 1.

Is this a bug, or is there some actual reason this should happen?

It's easily reproducible in Playgrounds like this:

import WebKit
import PlaygroundSupport
let cookie = HTTPCookie(properties: [.path: "/", .name: "foo", .value: "bar", .domain: "foobar.com", .version: 0])!
let webView = WKWebView()
print("My cookie is named \(cookie.name) and its version is \(cookie.version)")
webView.configuration.websiteDataStore.httpCookieStore.setCookie(cookie) {
    webView.configuration.websiteDataStore.httpCookieStore.getAllCookies { cookies in
        let transferredCookie = cookies.first!
        print("Transferred cookie is named \(transferredCookie.name) and its version is \(transferredCookie.version)")
    }
}
PlaygroundPage.current.needsIndefiniteExecution = true

The output is this:

My cookie is named foo and its version is 0

Transferred cookie is named foo and its version is 1