I'm not all that familiar with actor migration, so i might be misinterpreting.
We've recently had some complaints on project:support_desk ( https://www.mediawiki.org/w/index.php?title=Topic:V6ka95f08v2c89yp ) about stuff going missing on upgrade involving users with id 0. I think the issue is that update.php doesn't run cleanupUsersWithNoId.php prior to running migrateActors.php. At a glance, I don't see why we wouldn't do this.