I have successfully migrated User's from Sitecore 8.2 to 9.01. I can see the user's in my new Sitecore instance(9.0.1).
I can even login with those users to Sitecore Experience Platform.
But when I am trying to log in for the same user's through the code it's returning "User Not Found"
1) User.Exists(username ) ---> false
Even if user is there in system