Session not persisting while upgrading from 3.0.4 to 3.1.7

I'm busy upgrading from 3.0.4 to 3.1.7 and having issues with session. I've tested this on PHP 7.0 and PHP 7.2
I've seen a few other issues here regarding session on PHP 7.x with late versions of CI, but not any with resolution it seems.
When I run this:

Code:

   $this->session->set_userdata('test', 'test');
   print ($this->session->userdata('test'));

it prints "test" just fine. Then when I remove the 

Code:

set_userdata

 and reload the page, it's empty.

Either sessions aren't being stored, or they are being cleared. What could this be?