I am using the MDBT50Q-1MV2 which has the nRF52840, and have burned the Adafruit Feather Express bootloader using a J-link and the Arduino IDE. The issue is that I have not been able to get the current below ~2.4mA and that is using the systemOff(); function. I am not sure if I am stuck in debugging mode and if I am, how do I get out of it? If its not in debugging mode, then I am also stuck as I have tried many different sleeping function and have not gotten any lower current values.
To get out of debug mode you just need to do a power reset of the chip. I would suggest to test the board with a very simple application for example the rtc example in the nRF5 SDK v17.1 located at \examples\peripheral\rtc. The example put the CPU to sleep and only wake up by the RTC interrupt to toggle a LED. If you have a nRF52 DK you can try to compare the current. It should be around 2-3 uA.