- name: Wait port 22 to be ready
  local_action:
    module: wait_for
    port: "22"
    host: "{{ ssh_host }}"
    search_regex: OpenSSH
    delay: "{{ ssh_delay_seconds | default(5) }}"
    timeout: "{{ ssh_timeout_seconds | default(300) }}"

 ---
- hosts: localhost
  vars:
    params_servers:
      - connection_name: connection1
        connection_port: 55
        connection_param: fast

      - connection_name: connection2
        connection_port: 56
        connection_param: silent
  tasks:

    - name: set_fact connection_name_list
      set_fact:
        connection_name_list: "{{ connection_name_list|default([]) + item.connection_name }}"
      loop: "{{ params_servers }}"

    - name:
      debug:
        msg: "{{ connection_name_list }}"
...

 {"msg": "Unexpected templating type error occurred on ({{ connection_name_list|default([]) + item.connection_name }}): can only concatenate list
 (not \"str\") to list"}