AX3500 watchdog failed in closed-loop speed mode

Why watchdog function does not shut off the motor in the closed-loop speed mode? is there some setting that I am missing?

i.e. when a command !A15 is send to the controller using the roborun console, then I physically disconnect the serial cable between the pc and AX3500, the motor continues to run! The motor stops when it is in the open loop mode.