Using the Ansible user module: when there is no group mentioned and if a group already exists with the same name as the user, Ansible uses the -N option (no user group).
Using this option might result in setting a group that we did not want.

For example, if the "redis" group already exists when the task is run, the user will be created with a group different than "redis" (mine was set to "users" by useradd)
This is due to the behavior of useradd. See the man page below.

Man page of useradd
-N, --no-user-group
   Do not create a group with the same name as the user, but add the user to the group specified by the -g option or by the GROUP variable in /etc/default/useradd.
   The default behavior (if the -g, -N, and -U options are not specified) is defined by the USERGROUPS_ENAB variable in /etc/login.defs.

Ansible user module reference:
https://github.com/ansible/ansible-modules-core/blob/devel/system/user.py#L297

=> The solution is to explicitely set the redis group in the task.
Changed redis_group variable to redis_user_group.