I believe this is unique to EMS/TM integrated contracts.

We have an early discounted plan that expires in April. We are running into problems with parents who selected that plan for their currently enrolled child, then enroll a new child after that plan is no longer available. They get an error on the last screen and can't sign their contract!

We worked with Support to extend the months for this discount plan to get one family through the process, but this is not a payment plan we would want to leave displayed past the time we offer it.

The easiest answer would be to allow parents to select independent plans for each child.