It sounds like it’s working as intended.
You subscribed to received notifications when someone follows that broadcaster you specified.
After the subscription was verified and changed to enabled status, someone followed that channel and you got a notification. If you look in the event field of the payload you’ll see who it was that followed.
I’m not sure what you mean about premature notifications? You will be sent one any time that any user follows the broadcaster you specified.
It seems like you’re confusing how EventSub works, it’s not a subscription between you, the developer, and that EventSub type, it’s a subscription between an app (that doesn’t represent a user) and the EventSub type. Your user account doesn’t factor in to it at all (unless you specify yourself as the broadcaster in the subscription, or trigger a notification by being a user that follows that channel).