New answers tagged claims
SPWeb.EnsureUser evaluates against Active Directory (Windows) and Forms-based Authentication Claim providers. Since you're using a SAML provider (regardless if the user is originating from Active Directory), there is nothing for SharePoint to attempt to resolve against. In other words, what you're seeing is by design.
There's only one account in AD but they end up being treated like two different user profiles within SharePoint. You're on the right track when you mention the claims prefix. Most likely, this site at one point used Classic authentication and was later changed to Claims authentication (or vice versa but moving a site 'backwards' from Claims to Classic ...
Top 50 recent answers are included