Rails 6.1 adds support to handle db:rollback in case of multiple database application.
Prior to this change, on executing db:rollback Rails used to rollback the latest migration from the primary database. If we passed on a [:NAME] option along with to specify the database, we used to get an error.
Check out the issue for more details.
Staring with Rails 6.1, we need to pass the database name along with db:rollback:[NAME] otherwise a RuntimeError is raised.