Pipelines failing for 5.x - ToolbarTest

After core change at https://www.drupal.org/project/drupal/issues/2973356, our \Drupal\Tests\devel\Functional\DevelToolbarTest::testCacheHeaders started failing. Pages are Dynamic Page Cache (DPC) is reporting UNCACHEABLE where it used to report MISS. I looked into it a it but could not find the cause. This test depends on toolbar module and the toolbar_disable_user_toolbar module, so I wonder if the Uncacheable comes from there. Devel's toolbar items use a lazy builder so they shouldn't poison the DPC.