I did a "git bisect" and tracked the issue down to this change: https://gerrit.wikimedia.org/r/501594 (T188327). Since then, the test case CognateIntegrationTest::testCreateDeleteAndRestorePageResultsInEntry fails. What this test case does is deleting and undeleting a page, and then checking if the information in the Cognate database is undeleted as well. It appears either the information is lost, or the test setup is incomplete.
Example for a test failure: https://gerrit.wikimedia.org/r/505310
13:52:55 1) Cognate\Tests\CacheInvalidatorTest::testJobIsQueued 13:52:55 Failed asserting that two variables reference the same object. 13:52:55 13:52:55 /workspace/src/extensions/Cognate/tests/phpunit/CacheInvalidatorTest.php:34 13:52:55 /workspace/src/extensions/Cognate/src/CacheInvalidator.php:30 13:52:55 /workspace/src/extensions/Cognate/tests/phpunit/CacheInvalidatorTest.php:38 13:52:55 /workspace/src/tests/phpunit/MediaWikiTestCase.php:427 13:52:55 /workspace/src/maintenance/doMaintenance.php:96