I updated to the latest org, and I am not sure if you are seeing a bug. You might need to fix the space between : and session in your example, and name the session something. Also, you need to make sure the directory exists. This example works as expected for me:
#+begin_src python :results output :session t :dir ./run
import os
cwd = os.getcwd()
print("dir: ", cwd)
#+end_src
#+RESULTS:
: dir: /Users/jkitchin/Box Sync/kitchingroup/jkitchin/journal/2019/04/05/run
There are still some subtle possible issues, e.g. you cannot change the :dir after you run this in a session. I am not sure why. Personally I prefer ipython for sessions.
John
-----------------------------------
Professor John Kitchin
Doherty Hall A207F
Department of Chemical Engineering
Carnegie Mellon University
Pittsburgh, PA 15213
412-268-7803