Session data lost between calls (Node/Express) - node.js

Question

Right now my problem boils down to two servers. One node server is just serving up my static assets (HTML/JS/CSS). Another server is acting as an api - it's a rudimentary start to a microservices approach. I'm using PassportJS to handle authentication and Redis as a session storage.
When I login, I can see that my session data is being stored in req.session, but in every subsequent request, that session data is gone. The PassportJS documentation suggests that as long as I'm calling passport.initialize, the req.session.user object is created upon every request (with session data stored in the user object by invoking passport.session). However, the req.session.user object does not exist, and the only time it seems to appear is after I call req.login (and by then it has the user's data stored).
This is my middleware implementation inside the API server:
app.use(cookieParser());
app.use(bodyParser.urlencoded({ extended: true }));
app.use(bodyParser.json());
app.use(session(sessionConfig));
app.use(passport.initialize());
app.use(passport.session());
An example of what req.session contains after req.login is invoked
{ cookie:
{ path: '/',
_expires: null,
originalMaxAge: null,
httpOnly: true },
passport: { user: <some user id> } }
What req.session contains on any request, including login prior to explicitly calling req.login
{ cookie:
{ path: '/',
_expires: null,
originalMaxAge: null,
httpOnly: true } }
Is there something I can do to salvage all this PassportJS code? This setup seemed to work fine prior to splitting the static asset handling into it's own server. It also seemed to work fine though when I was using NGINX (which I got rid of because of my lack of comfort with a non-scripting tool) to serve up my static assets instead of a node server.
I have an inclination that I'll probably have to roll my own authentication and explicitly pass the sessionid every time I make a request from the client, but I'd prefer not to in case I'm missing something here.