I followed the documentation about multi-tenant AAD external identy providers and created a test bed with a main tenant with AAD Connect (to provide access to internal users) and two secondary tenants, each with a private AAD B2C (to provide access to external customers).
I registered both the secondary tenants on the main tenant by creating two App registrations, each with its own client certificate, and enabled the custom policy framework on one of the two secondary tenants. With this configuration I'm able to complete the SignUp flow of a new user on one of the AAD B2C (the one without custom policy) but when I try to login with this account on the second AAD B2C, i.e. when I try to login with the custom flow that I have exposed on the login form, I'm redirected to the Microsoft login page and I can only provide my corporate credentials, not the credentials of the customer registered on the first tenant.
Any idea about the reason of this unexpected behavior?