Jinja2: nested for loop where inner loop is determined by outer

group_vars_all.yml

haproxy_listener:
    - name: api
      port: 6443
      group: kube-master

    - name: http
      port: 80
      group: kube-master

    - name: https
      port: 443
      group: kube-master
      

inventory.ini

localhost   ansible_hostname=127.0.0.1 ansible_connection=local


[load-balancer]
kube-00     ansible_hostname=10.0.0.10

[kube-master]
kube-01     ansible_hostname=10.0.0.11
kube-02     ansible_hostname=10.0.0.12
kube-03     ansible_hostname=10.0.0.13

[kube-node]
kube-04     ansible_hostname=10.0.0.14
kube-05     ansible_hostname=10.0.0.15
kube-06     ansible_hostname=10.0.0.16

[k8s-cluster:children]
kube-master
kube-node

loadbalancer.yml

---

- hosts: load-balancer
  
  gather_facts: false

  tasks:
    - name: expand
      set_fact:
        val: |
              {% set ns = namespace(v=[],srv=[]) %}
              {% for i in range(haproxy_listener|length)|list %}
              {%   set entry = haproxy_listener[i] %}
              {%   set ns.srv = [] %}
              {%   set group = entry.pop('group', None) %}
              {%   for host in groups[group] %}
              {%     set ns.srv = ns.srv + [ { 'host': hostvars[host].ansible_hostname, 'port': entry.port}] %}
              {%   endfor %}
              {%   set ns.v = ns.v + [ haproxy_listener[i] | combine({ 'i': i, 'servers': ns.srv }) ] %}
              {% endfor %}
              {{ ns.v }}

    - name: test
      debug:
        msg: "{{ (val | to_nice_json).split('\n') }}"
      delegate_to: localhost
      run_once: true

The haproxy_listener[i].group key will be replaced with a haproxy_listener[i].servers array of dictionaries populated based on the groups[group] inventory host group.