Lineinfile sometimes doesn't replace lines

I have nginx upstream with multiple backends. I use pre_tasks in playbook for disabling backends in the upstream config during deployment:

- name: Deploy
  hosts: '{{ list_hosts }}'
  serial: 4

  pre_tasks:

  - name: Disable hosts in nginx upstream
    replace:
      path: /etc/nginx/conf.d/upstream.conf
      regexp: '^        server {{ LAN }}:'
      line: '        #server {{ LAN }}:5001;'
    delegate_to: "{{ item }}"
    with_items: "{{groups['nginx_api']}}"
...

It works, but not always correct. Sometimes it disables not all 4 hosts (serial: 4) I think it happens because it tries to replace 4 lines at the same time.

Is it possible to do each iteration one by one and keep serial: 4?

Update:

Sorry, my last test was with replace

To regexp added ^ (thanks @bviktor)

- name: Deploy
  hosts: '{{ list_hosts }}'
  serial: 4

  pre_tasks:

  - name: Disable server in nginx upstream
    lineinfile:
      path: /etc/nginx/conf.d/upstream.conf
      regexp: '^        server {{ LAN }}:5001;'
      line: '        #server {{ LAN }}:5001;'
    delegate_to: "{{ item }}"
    with_items: "{{groups['nginx_api']}}"

In inventory

[nginx_api]
api1 ansible_host=x.x.y.1   LAN=10.x.y.1
api2 ansible_host=x.x.y.2   LAN=10.x.y.2
api3 ansible_host=x.x.y.3   LAN=10.x.y.3
api4 ansible_host=x.x.y.4   LAN=10.x.y.4
[app]
app1 ansible_host=x.x.x.1   LAN=10.x.x.1
app2 ansible_host=x.x.x.2   LAN=10.x.x.2
app3 ansible_host=x.x.x.3   LAN=10.x.x.3
app4 ansible_host=x.x.x.4   LAN=10.x.x.4
app5 ansible_host=x.x.x.5   LAN=10.x.x.5
...

Upstream conf before run:

upstream app {
least_conn;
        server 10.x.x.1:5001;
        server 10.x.x.2:5001;
        server 10.x.x.3:5001;
        server 10.x.x.4:5001;
        server 10.x.x.5:5001;
    ...

}

Upstream conf after run:

upstream app {
least_conn;
        server 10.x.x.1:5001;
        #server 10.x.x.2:5001;
        server 10.x.x.3:5001;
        server 10.x.x.4:5001;
        server 10.x.x.5:5001;
    ...

}

Expected result:

upstream app {
least_conn;
        #server 10.x.x.1:5001;
        #server 10.x.x.2:5001;
        #server 10.x.x.3:5001;
        #server 10.x.x.4:5001;
        server 10.x.x.5:5001;
    ...

}

Just when I run the playbook several times it disables 4 needed hosts in upstream

You give us very little to work with, but in general, it's not a very good practice to replace a pattern with something that is a superset of such pattern. Why? Because on consecutive runs, it will match over and over, since it includes the original pattern. So server {{ LAN }} becomes #server {{ LAN }}, then ##server {{ LAN }}, then ###server {{ LAN }}, and so on. Use ^ or some other pattern to indicate the preceding pattern. For example, if you use something like

regexp: '^server {{ LAN }}:'

It will only match if the line starts with server, i.e. it doesn't have any # preceding it, and therefore it won't put any more # characters before it upon consecutive runs.

The solution is to use a throttle

- name: Deploy
  hosts: '{{ list_hosts }}'
  serial: 4

  pre_tasks:

  - name: Disable server in nginx upstream
    lineinfile:
      path: /etc/nginx/conf.d/upstream.conf
      regexp: '^        server {{ LAN }}:5001;'
      line: '        #server {{ LAN }}:5001;'
    delegate_to: "{{ item }}"
    throttle: 1
    with_items: "{{groups['nginx_api']}}"