Steps to reproduce:
- use Chrome in incognito mode (with X-Wikimedia-Debug header until SessionManager is deployed everywhere)
- register a new account on enwiki
- log in with it on meta
- visit dewiki
Expected: you are logged in on dewiki.
Actual: you are logged out on all three wikis.
All steps are necessary. If you load another page on meta or enwiki before visiting dewiki, you won't get logged out on that wiki.