Visiting one Django server logs me out of another Django server, both behind the same proxy

I have two Django servers A1 and S1, which sit behind a simplistic NodeJS proxy. This is a silly attempt at single sign on.

I can log into and out of A1 (authentication server) just fine. If I log into A1, visit S1 (without being logged in to S1) and then revisit A1, I am no longer logged in. The S1 server doesn't set a new session ID in the cookie and I don't think from memory that the CSRF changes. The session ID cookie hasn't changed, the domain is the same etc.

I can't work out why I'm no longer logged into A1. I know this isn't much to go on but I'm assuming something is happening to the cookies set by S1 when I visit it. Any suggestions appreciated.

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