SBLM2360T velocity commands not persistent in closed loop speed mode

I am trying to set closed loop speed target velocity using the serial commands !S and !G. In both cases the motor will start to spin then stop. I do not see any sign of errors or loop failure to explain the stop. If I continuously send the command the motor spins as expected. I am expecting a functionality closer to the slider in roborun where I set a speed and the motors continue to spin at that speed till I set a new speed. If this is not how the !S and !G commands are designed to work in closed loop speed mode is there another closed loop mode of operation that I could use that would not require constantly sending the desired speed?

The serial port has a watchdog timer, if you don't keep sending commands when the timer expires it stops the motor.  You can disable it or change the length of time in the settings.