Android FirebaseAuth.getCurrentUser() Never Null

Firebase Authentication will cache an authentication token for the user when they're signed in. This prevents having to authenticate every little interaction the user makes with other services provided by Firebase. This token gets automatically refreshed periodically, but until then, the SDK assumes that the token represents the user. You'll find that the token will expire after some time, or you can force the issue by uninstalling and reinstalling the app, or clearing its data.

Authentication works regardless of the contents of your Realtime Database. They are independent from each other, except where security rules limit access to the currently authenticated user.