Which directory are you in when the cookie gets set?
From the PHP manual on setcookie(), emphasis mine:
Path
The path on the server in which the cookie will be available on. If set to “https://stackoverflow.com/”, the cookie will be available within the entire domain . If set to ‘/foo/’, the cookie will only be available within the /foo/ directory and all sub-directories such as /foo/bar/ of domain . The default value is the current directory that the cookie is being set in.