POF comparator doesn't work as expected- seems to crash chip

Question

Hardware: nRF52840, running on dev kit PCA10056

Software: SDK 15.0.0, no soft device

I'm trying to get the POF comparator working, and after struggling in our product I pared right back to a test case with blinking leds on the dev kit. (This dev kit has power input over P21). Running the following code, without USB attached, and gradually reducing the voltage supplied to P21 exhibits the problem:

#include <stdbool.h>
#include <stdint.h>
#include <string.h>
#include "nrf_delay.h"
#include "nrf_gpio.h"
#include "nrfx_power.h"

#define LED_2 NRF_GPIO_PIN_MAP(0,14)
#define LED_4 NRF_GPIO_PIN_MAP(0,16)

static void pof_event_handler(void);

static volatile uint32_t pof_events;

int main(void)
{
    nrf_gpio_cfg_output(LED_2);
    nrf_gpio_cfg_output(LED_4);

    nrf_gpio_pin_write(LED_2, 0);

    const nrfx_power_pofwarn_config_t config = {
        .handler = pof_event_handler,
        .thr = NRF_POWER_POFTHR_V24,
        .thrvddh = NRF_POWER_POFTHRVDDH_V27
    };
    const nrfx_power_config_t power_config = {
        .dcdcen = false,
        .dcdcenhv = false
    };
    NRF_CLOCK->TASKS_HFCLKSTART = 1; // Is this needed?

    nrfx_err_t rc;

    rc = nrfx_power_init(&power_config);
    ASSERT(rc == NRFX_SUCCESS);
    nrfx_power_pof_init(&config);
    nrfx_power_pof_enable(&config);

    for (;;)
    {
        nrf_delay_ms(1*1000);
        nrf_gpio_pin_write(LED_4, 1);
        nrf_delay_ms(1*1000);
        nrf_gpio_pin_write(LED_4, 0);
    }
}

static void pof_event_handler(void)
{
    nrf_gpio_pin_write(LED_2, 1);
}

Without the call to nrfx_power_pof_enable this code continues blinking LED 4 (faintly) all the way down to 1.61 V, then cuts out both leds and reboots into flashing when power gets above 1.62V again.
With the line nrfx_power_pof_enable this code blinks LED 4 until 2.36V, then stops blinking the led, does not turn off led 2, as the pof event handler should, and does not restart when power goes up to 3.3 again.

It seems that POF crashes the chip when it fires.

How can we get POF detection working?