Reset button doesn't work on some nRF52840 DK PCA10056 1.0.0 boards

The reset button doesn't seem to work on two of my PCA10056 1.0.0 development kits, but works on two others, and works on some earlier versions.

I tested by running the peripheral/uart and peripheral/blinky examples with SDK 15.0.0 and SDK 15.2.0

nrfjprog -f NRF52 -r works fine.

What could I be doing wrong to make the reset button not work on two specific boards? I'm not modifying the examples to not define CONFIG_GPIO_AS_PINRESET or anything.