Issue re-using variables in vars_prompt

[Matt SF]

Hello,

The following playbook extract used to work with Ansible 2.4 :

---
- hosts: localhost
  gather_facts: no
  vars_prompt:
    - name: "user_password"
      prompt: "Shadow password"
      private: no
    - name: "user_password_prod"
      prompt: "Shadow password for prod (Enter to keep the same)"
      private: no
      default: "{{ user_password }}"

Now using Ansible 2.7.8 I get an error :

ERROR! 'user_password' is undefined

The playbook does not even prompt me at all.

Is there a workaround for this ? How else can I re-use a previous variable as a default in a prompt ?

Thank you for your help.

Mathieu.