Question about admin role Does the admin role always has access to everything ?

Because even if my AccessResult returns false, my admin still have acces to it (but it's perfectly working for the other roles)
Not the admin role, no. But user 1 does have access to everything, yes.

This is the function responsible, User::hasPermission, you can see the unconditional access check at the top:

public function hasPermission($permission) {
    // User #1 has all privileges.
    if ((int) $this-&gt;id() === 1) {
      return TRUE;
    }

    return $this-&gt;getRoleStorage()-&gt;isPermissionInRoles($permission, $this-&gt;getRoles());
  }