Can't sign in to my app's Sandbox on Xbox One in dev mode

• My dev account was accepted into the Xbox Live Creators Program.
• I made a UWP app in my Dev Center Dashboard.
• I enabled the Xbox Live service, and got a Sandbox ID.
• I added my dev account and my personal account to the authorized Xbox Live test accounts. I clicked the "Test" button to publish the changes.
• On my Xbox One in dev mode, I changed the Sandbox to the app's Sandbox ID. It restarted the Xbox.

When I try to sign in to either my dev account or personal account, I get the following error:

Use an account with access to this sandbox

Either the account or title can't use this sandbox. Change the sandbox in
Settings > Console > Developer settings, or use a different account.

I can't even launch the app unless I'm signed in. Am I missing any steps?

In the "XDKS.1" Sandbox, I'm able to sign in, deploy and launch the app, etc. But within the app, I'm unable to sign into Xbox Live, and I'm assuming it's because I'm not in the app's Sandbox.

I got in touch with an Xbox Live engineer, and after some back and forth, determined it was a problem with my Sandbox's state on their end. They fixed it for me, and now it's working.

So the above 5 bullet points are the correct steps to start developing a UWP app for the Xbox Live Creators Program.