I've been documenting this issue and asking questions here:
Intermittent device reset in modified ble_app_beacon example
Since I need to post a proprietary schematic, I made this private post to continue.
This project is a stripped down version of a product under development. It depends upon being able to change the contents of the advertising data whenever the board (a sensor) detects a new state. The code as shown generally works but eventually the device resets. This could happen after running for a few minutes or a few days or anywhere in between.
I've removed the schematic and am now making this post public.
At this point I am gaining confidence that the programmer was damaging the coin cells with the reverse current. I've appealed to Nordic to document this in the DK user guide and am converting this post to public in case they don't.
The schematic shows C7 as DNP. That has changed, it is now a 680uF electrolytic.