Shield Login possible on active = 0

Hey guys,
as the title says, its possible to login with Shield although active flag is set to 0 in users table.
I can activate/deactivate user with the corresponding functions ($user->activate() / $user->activate() ), the status in the database gets updated correctly but the function $user->isActivated() always returns true.
My version is 4.6.0
There are several existig threads about this bug, but none of them addresses the problem.
Thanks!