iOS 12 app autofill and app association

Is there a way to associate an app for an item in your vault? If not, I would like to see that added.

No, this is something that the app itself has to do.

https://developer.apple.com/documentation/security/password_autofill/setting_up_an_app_s_associated_domains

So basically you’re telling me that I’m at the mercy of the app developer. Right?

Yes, unfortunately. This is the way iOS 12 Autofill works.