sd_power_system_off returns immediately and forces reset

I hope this isn't a duplicate of another question. I searched on sd_power_system_off and didn't see anything exactly like this.

I'm creating a new project, based on ble_app_template from SDK 11.0.0 . So far, all I've done is tested to make sure my combination of services advertises correctly. It does, but I noticed via debug output that the application restarts regularly and I tracked it to the sleep_mode_enter call (due to advertising timeout) and sd_power_system_off. The latter call is made and immediately exits with error NRF_ERROR_SOC_POWER_OFF_SHOULD_NOT_RETURN.

What might cause this?

I'm attaching my main.c. main.c

ble_whts.c ble_whts.h