Some customers may experience issues adding users to RLS roles on the RLS Security tab. As a potential workaround, customers can refresh the page and add the user. Engineers are working on a fix and expect it to be deployed to all regions by end-of-day 3/29/19.
It irritates me that on an enterprise class product like this, they break stuff and go "Well, sorry about that. Hope to resolve in 8 days or so."