ID not showing in the parameter - asp.net-mvc

Question

I have a code that sends email to user. In the email, there's a link in there that they should visit with the corresponding ID in order for them to be directed to a certain page.
Here's my code:
public void Notify(int appr_id = 0)
{
var check = db.rms_approval_routing.Where(s => s.appr_id == appr_id && s.appr_isactive == true).FirstOrDefault();
try
{
if (check != null)
{
check.status_id = 8;
db.Entry(check).State = EntityState.Modified;
db.SaveChanges();
var getinfo = db.rms_approval_route_vw.Where(s => s.appr_id == appr_id && s.appr_isactive == true).FirstOrDefault();
var getpayment = db.rms_payment.Where(s => s.appr_id == appr_id).FirstOrDefault();
if (getinfo != null && getpayment != null)
{
var ref_email = getinfo.ref_email;
var cc_email = getinfo.user_email;
var pay = getpayment.p_amount;
var body = "";
body = "Good day!<br><br>Please be informed that you have successfully referred <u>" + getinfo.Fullname + "</u> and you are entitled to receive <u>P " + pay + "</u> which will be credited on the next payout for your successful referral.<br>Kindly visit the link to acknowledge the payment: http://localhost:8119/ReferralLetter/Acknowledge/" + appr_id + " <br>Thanks!";
SendEmailController email = new SendEmailController();
email.SendReferrer(ref_email, cc_email, body);
}
}
}
catch (Exception)
{
throw;
}
}
public ActionResult Acknowledge(int appr_id = 0)
{
var check = db.rms_emails.Where(s => s.appr_id == appr_id && s.email_date_ack == null && s.email_isactive == true).FirstOrDefault();
if (check != null) {
ViewBag.email_id = check.email_id;
ViewBag.appr_id = appr_id;
return PartialView();
}
return RedirectToAction("Denied");
}
In this line: http://localhost:8119/ReferralLetter/Acknowledge/" + appr_id
The appr_id value is 0 when I tried to breakpoint the Acknowledge function. When I received the email, it showed there this line: http://localhost:8119/ReferralLetter/Acknowledge/23
Meaning there's an ID in there but why in the Acknowledge function the ID was 0?