The PCA9685 datasheet states under heading "7.3.4 ALL_LED_ON and ALL_LED_OFF control" the following: "The LEDn_ON and LEDnOFF counts can vary from 0 to 4095. The LEDn_ON and LEDn_OFF count registers should never be programmed with the same values."
As an experiment, we set LEDn_ON to 0 and LEDn_OFF to 0, and the result was 0 output, the same as setting bit4 of the LEDn_OFF_H register. So, it is almost as if the Full Off bit is not required.
However, to get full on, bit4 of the LEDn_ON_H register does need to be set to get full output. Otherwise, you get 1 cycle of off condition for "FFF".