This test demonstrates the issue - the final step will fail because when a user is already logged in, the next log in attempt clears the session and with that the big pipe cookie is deleted.
Scenario: Assert that Big Pipe cookie is set when there are multiple user sessions in a scenario.
Given I install a "big_pipe" module
When I am logged in as a user with the "administrator" role
Then cookie "big_pipe_nojs" exists
Then I am logged in as a user with the "authenticated" role
Then cookie "big_pipe_nojs" exists
This test demonstrates the issue - the final step will fail because when a user is already logged in, the next log in attempt clears the session and with that the big pipe cookie is deleted.