When using alternate flap extension the LE flaps and slats are driven to the full extended position using power from the standby hydraulic system. In this case the ALTERNATE FLAPS master switch energizes the standby pump and the ALTERNATE FLAPS position switch, held in the down position momentarily, fully extends the LE devices.
Note: The LE devices cannot be retracted by the standby hydraulic system.
In a nutshell, the PTU becomes isolated along with the hydraulic system to prevent hydraulic lock, as mentioned before, the PTU is only for autoslat operation
I'm guessing in the sim you had the APU on the bus and the load shedding of the electrical hydraulic pumps didn't occur