Can't see why this is occurring.
I've checked my custom board and the reflow of the chip under microscope. The doc indicates this pin isn't linked to a peripheral. Despite this the pin is fixed at 3.3v output.
#define CFG_PIN_FRONT_EN 8
NRF_GPIO->PIN_CNF[CFG_PIN_FRONT_EN] = (GPIO_PIN_CNF_DIR_Output << GPIO_PIN_CNF_DIR_Pos);
NRF_GPIO->OUTSET = 1 << CFG_PIN_FRONT_EN; // no effect
NRF_GPIO->OUTCLR = 1 << CFG_PIN_FRONT_EN; // no effect
