The test for CheckUserPrivateEventLogFormatterTest::testLogDatabaseRowsForHiddenUser has the log row expandDatabaseRow() with only the target user name.
In this test, the row represents an event performed by the hidden target user. The test creates target user, hides it, and then expects CheckUserPrivateEventLogFormatter to suppress the target user name. Not setting the user and log_actor in the function makes expandDatabaseRow() fallback to different actor different from the actor row in the test database.
Today the formatter checks the hidden database user and returns the expected suppressed name but changing User::load() function to leverage ActorStore as per task https://phabricator.wikimedia.org/T428517 produce a mismatch, two different users with the same name:
ActorStore: id=0, actorId=24, hidden=false SELECT actor: id=1, actorId=1, hidden=true ActorStore: userId: 0 actorId: 24 name: "TestUser MediaWikiIntegrationTestCase 963954.001" SELECT actor: userId: 1 actorId: 1 name: "TestUser MediaWikiIntegrationTestCase 963954.001"