When using an Azure AD/ADFS connection with HRD and the Identifier First authentication profile, if a user tries to SIGN UP with an email domain configured in Home Realm Discovery, they are redirected to sign up for a DB connection rather than redirecting the user to login with the IdP configured for that domain.
It works as expected for other Enterprise connections, like SAML or OIDC.
We have a backlog feature request to use the same criteria for all Enterprise connections.
As a workaround, you could create a SAML or OIDC connection to your Azure AD tenant.
Here are the instructions for creating those Enterprise connections: