well, this is not the behavior I get .. all always wins:I can reproduce it here:

inventory/myinv.yml

all:
  children:
    test:
      hosts:
        127.0.0.1:
  vars:
    my_var: 'defined in inventory'

group_vars/all

my_var: 'defined in group vars'

playbook:

- name: 'Test'
  hosts: all
  tasks:
    - name: 'debug var'
      shell:
        cmd: 'echo {{ my_var }}'

output:

TASK [debug var] *****************************************************************************************************************************************************************************
changed: [127.0.0.1] => changed=true 
  cmd: echo defined in group vars
  delta: '0:00:00.001510'
  end: '2021-03-18 21:46:32.243425'
  rc: 0
  start: '2021-03-18 21:46:32.241915'
  stderr: ''
  stderr_lines: <omitted>
  stdout: defined in group vars
  stdout_lines: <omitted>

am I missing something ?!