Values of $application.user.userId system variable changed unexpectedly?

Summary:

The value of the system variable $application.user.userId unexpectedly changed from:

firstname.lastname@example.com (an IDCS username, basically).

To:

idcs-<32 alpha numeric characters>/firstname.lastname@example.com

Has anyone noticed this? Could anyone tell us what caused this change?

Content (required):

We've been building a web application using Visual Builder Studio as part of our OIC instance. In our web app, we've been using the value of the system variable $application.user.userId to identify the authenticated user. We also have a custom back end in which we store some additional data for our users. This additional data can be retrieved using that same userId.