Customer is creating on Stripe end and should be attached to a session. The last one is created after a buyer filled in the payment form. Then session with customer and the other info should be returned to UNA for further usage. If it wasn't returned we cannot do anything on UNA end. May be you have some errors on Stripe end. Did you check events related to your payments in Stripe account?

Also what integration you are using: old one or new one with 3D Secure? If you have new Stripe account then Old integration may work incorrectly.