Hi there, and Welcome
There is an exploit, Are if you are using the BSP 2.9.2
You can call the system sleep function and not use a delay and with the interrupt pin don’t use the gpio pin number. 
The watchdog can call the MCU sleep and it will briefly (ms.) then reset.
or have the watchdog call the SoftDevice system reset maybe?
HTH
GL 
PJ
look up, the 2.9.2 breaks interrupt ithink