SetCookie() couldn't set dot prefix for domain

Question

package main

import (
    "time"

    "github.com/gin-gonic/gin"
)

func main() {
    r := gin.Default()
    r.GET("/ping", func(c *gin.Context) {
        c.SetCookie("NAME","VALUE",23*60,"/",".a.b.com",true,true)
        c.JSON(200, gin.H{
            "message": "pong",
        })
    })
    r.Run(":8089") // listen and serve on 0.0.0.0:8080 (for windows "localhost:8080")
    time.Sleep(60*60)
}

then I executed ❯ curl -v GET 'http://0.0.0.0:8089/ping'

I hope it returned with Set-Cookie: NAME=VALUE; Path=/; Domain=.a.b.com; Max-Age=1380; HttpOnly; Secure The domain value with a dot prefix.

But it actually returned: Set-Cookie: NAME=VALUE; Path=/; Domain=a.b.com; Max-Age=1380; HttpOnly; Secure

I don't know why.