Problem/Motivation
Still figuring this out, but #3044155: PHP7.1 vendor max Testing issue - DO NOT COMMIT identified test failures with symfony 3.4.24, released today, 2019-04-02.
https://www.drupal.org/pift-ci-job/1249248 shows the failing tests.
I've tracked it down to session handling, and specifically this commit: https://github.com/symfony/symfony/commit/029fb2e7e36b7cdf29e27d4bfa54dd...
seems our session isn't showing as started, so it isn't getting saved.
Proposed resolution
Discussed with mikelutz and mglaman in slack, we might need to override the SessionManager->isStarted() to be aware of our lazy session handling.
Remaining tasks
User interface changes
API changes
Data model changes
Release notes snippet
Fixed session handling with Symfony 3.4.24