All SLCD controllers have a reset circuit triggered when the voltage supplied to the board drops below a particular threshold. Sometimes called brown-out protection, this trigger ensures the controller’s processor does not incorrectly execute and possibly change its internal programming when it loses power. This reset happens when the input voltage to the controller sags (dips down below 10% of nominal).
The display backlight draws a lot of power, and when it turns on, the controller input voltage can sag and cause the controller to reset and turn off the backlight. This can make it look like the display is flashing since it turns on the backlight and then resets and keeps doing this as long as power is applied. The solution is to shorten the cable between the power source and the controller or use larger cable wires. Long thin power cables have inductance which can cause voltage sags when there is a change in current. To see the problem, put an oscilloscope on the controller power input and set the trigger at 10% less than the nominal input voltage.