When using my facebook developer administrator account, after clicking a One-time notification - "Notify Me" button I receive the callback message_optins webhook from facebook with the one_time_notif_token. When using a tester/public account, I don't receive this callback to my webhook, but can do every other type of messaging.
This makes me feel like it would be some kind of permissions issue, but I have the MyPage -> Advanced Messaging -> Requested Features -> One-Time Notification enabled. Any suggestions why this would be?
It turns out I was using Facebook.com/messenger, which has been buggy for not returning the optins webhook request (see https://developers.facebook.com/support/bugs/310445676685298/). Using the mobile messenger app solved my issue.