Looks like we regressed in https://chromium-review.googlesource.com/c/chromium/src/+/776814 almost a year ago. Surprisingly we don't hear any complaints...

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/092e11d3114b336a3690833d5c802736c9efc166

commit 092e11d3114b336a3690833d5c802736c9efc166
Author: John Abd-El-Malek <jam@chromium.org>
Date: Thu Oct 18 22:20:42 2018

Fix GAPS cookie not being sent during reauthentication.

Manual repro steps:
1) start linux_chromeos's chrome with:
     --user-data-dir=/tmp/chrome --login-manager
2) login and close chrome
3) in "/tmp/chrome/Local State", replace OAuthTokenStatus's value from 4 to 3
4) start chrome again with same command line parameters and sign-in
5) the outgoing network request should have a Cookie header

Bug:  896862 
Change-Id: I79b96ba071c3485578223a1a16ef6a48350c760f
Reviewed-on: https://chromium-review.googlesource.com/c/1289674
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600940}
[modify] https://crrev.com/092e11d3114b336a3690833d5c802736c9efc166/chrome/browser/resources/gaia_auth_host/authenticator.js

Interesting, sorry for breaking this :-) Did we have tests which should have failed, or should we add tests we should have had?

Yes, issue 896863 is filed to add a test to cover the case.

Oh, I was just blind. Thanks!