Request was routed from a verified domain which is not CustomUrlDomain

I have a front door setup that has been working for many months. Now all of a sudden I am getting: **"**AADSTS399280: Request was routed from a verified domain which is not CustomUrlDomain."

Any ideas? I cant see any changes at all to any of the configuration or setup. This was working on Friday, and now on Monday its not.

The Setup:

I have a production Entra for my company that is on domain.com. To allow folks to login to my website on domain.com I have a second Entra for Customers configured to use my.domain.com. My website (Wordpress) then authenticates with mydomain.ciamlogin.com... I have a Front Door setup in ProdEntra to allow auth on login.domain.com for mydomain.ciamlogin.com.

The Problem: The route to login.domain.com results in "AADSTS399280: Request was routed from a verified domain which is not CustomUrlDomain." error.

Workaround:

On the auth, if I swap out the domain I can get it to work, so this does not work:

https://login.domain.com/658c1a65-f834-4a61-b64c-91b9a7b3e481/oauth2/v2.0/authorize?client_id=f18a31b9-50fb-45ea-ae5a-d904d0b19c67&nonce=66782fc4c344d&redirect_uri=https%3A%2F%2Fdomain.com%2Fauth%2F&response_mode=query&scope=https%3A%2F%2Fgraph.microsoft.com%2Fuser.read+openid+email+profile&state=https%253A%252F%252Fdomain.com%252Fwp-admin%252F&response_type=code

But this does:

https://mydomain.ciamlogin.com/658c1a65-f834-4a61-b64c-91b9a7b3e481/oauth2/v2.0/authorize?client_id=f18a31b9-50fb-45ea-ae5a-d904d0b19c67&nonce=66782fc4c344d&redirect_uri=https%3A%2F%2Fdomain.com%2Fauth%2F&response_mode=query&scope=https%3A%2F%2Fgraph.microsoft.com%2Fuser.read+openid+email+profile&state=https%253A%252F%252Fdomain.com%252Fwp-admin%252F&response_type=code

Questions: I would like to understand what the problem is, and how to fix it.

Has something changed in the way Microsoft configures these features?

Where do I find the CustomUrlDomain field?