Recent auth change has broken partner account app access

Background context: I created a custom app years ago and have been using it to integrate with all of my managed partner accounts:

This morning, I updated the app, and my code, to be compliant with the new auth changes. I then clicked "Login" for a partner and reauthorized the new app. This worked without error.

The bad news is that when I make a request to /account/summary it returns data for the master account, not the partner account. This is disastrous for us; and is a change from previous behavior.

I need help urgently on this please.