I've got a user who is reporting an issue:

The app is asking him to log in, even though they're already logged in to StackExchange. Regardless of how many times they log in, the message is the same.
The auth URL I'm using is:
If I click this link I'm taken to the "correct" authorization screen, and if I've previously authorized the app, I'm immediately redirected.
I've also sent the user to a different tool's access_token generation page, and they get this same loop.
What could be going wrong here?