Realm v12 throwing "Error: Cannot determine which app is associated with user"

I’ve just upgraded my Realm v11 app to the alpha of v12 to try it out and (I believe) followed the instructions in the CHANGELOG to bring the app up to date.

I log in a user with:

const creds = Credentials.emailPassword(username, password);

However, the UserProvideris throwing an error saying:

Error: Cannot determine which app is associated with user (id  = <correct user id>)

Did I miss a step in the migration?