--start-at-task does not work with group-by groups, at least not for group-by ansible_os_family If hosts is one dynamically generated by group-by, --start-at-task="common | Task1" will skip all tasks. if hosts is changed to something in hosts file, start-at-task works.

main.yml

---
# roles/common/tasks/main.yml

 - name: Task1
   command: echo task1

 - name: Task2
   command: echo task2

site.yml

- hosts: all
  tasks:
     - group_by: key={{ ansible_os_family }}

- hosts: group_defined_in_hosts_file
#- hosts: RedHat
  gather_facts: False
  vars_files:
    - "vars/{{ansible_os_family}}.yml"
  roles:
     - common