Earlier quoted context omitted.
A login can have multiple URLs. For sites which don’t automatically load the right entry, you can add another URL to give 1pw a hint. This won’t solve all your problems. It won’t even solve the problem you describe the first time you encounter it. Nor will it solve it for apps that fail to provide an INTENT URL. But hopefully it will make things a little easier.
That would improve the completion, but ideally 1Password should allow me to select the login myself within the app modal (by navigating to "all logins" with the filter deactivated), and then add the intent URL for me.
Agreed. You should consider posting to their forum. I have for several issues, and they have been responsive and helpful.