Multiple Spotify Accounts: Documentation when followed doesn't work, what am I doing wrong?

I have followed the instructions regarding adding additional spotify accounts, but I continually receive this error when attempting to authorize:

INVALID_CLIENT: Invalid redirect URI

Here’s what I’ve done.

  1. I have a Premium family account. I use account 1 with my personal email address.
  2. I have added a 2nd family spotify user reserved for Home Assistant to use so HA doesn’t hijack my iPhone when I’m out and about listening to music.
  3. I have created an app under that 2nd login and have authorized HA to control Spotify using that account. This created a media_player.home_assistant_spotify entity.
  4. I have added my first account to that app under the 2nd account’s login as an authorized user.
  5. I log out of all instances of Spotify.
  6. I open an incognito browser and login to Home Assistant.
  7. I go to the existing Spotify integration and click “Add Service” and I’m taken to the Spotify login.
  8. I login with my personal Spotify account (account 1).
  9. INVALID_CLIENT: Invalid redirect URI is the result.

Am I doing something wrong?