Cloudflare Access Custom Pages - Unable to change team domain

We are experiencing an issue where we cannot change the Cloudflare Access team domain from (in our case) polykey.cloudflareaccess.com to matrixai.cloudflareaccess.com, as it gives the error “This authentication domain has already been claimed”.

Why is this happening, and how can we fix it?

The authentication domain you are trying to use is in use by another account. You will need to select another or stick with the one you have.