This may or may not help you, but it is worth trying.
I had my email hacked last year which prompted me to change the email I had attached to my gamertag. After the change, I noticed that I couldn’t use my new email to log in to Waypoint, I would get a message saying “gamertag is already in use.” During that time I was forced into using my old email address to log into Waypoint.
However, like you, I recently changed my gamertag. Afterwards, I noticed that I could log into Waypoint with my new email account and that my new gamertag appeared.
I think this is where the problem lies. In order to see your new gamertag on Waypoint, you might need to attach a new email account to your gamertag. I don’t think that the Waypoint login system has the ability to notice if you changed your gamertag. As long as you keep using your old email account to login, Waypoint will show your original gamertag.
Basically what you need to do is create a new Waypoint account. Attach a new email account to your gamertag, and create a new account using that email. Once you are logged in, your new gamertag should be visible.