Unfortunately the Kirby 5.3.0 release introduced a regression in the user handling that leads to wrong results when using $users->find(), $field->toUsers() or $field->toUser(). All of these wrongly return the entire set of users without filtering.
We already have a fix and plan to release 5.3.1 later today. Please hold off from an upgrade to 5.3.0 until then.
Sorry for the inconvenience and thanks for your understanding.