Yes, there is miniscule probability of this happening if the 1.8V regulator output (input power to the on-die LDOs, which make 1.2V VDD_CORE) goes below about 1.6V. But the RTC POR is by default set to only give an RTC reset if VRTC falls below 1.4V. So there is a voltage range where a dip could cause CORE BROWNOUT. Our module is not 100% brownout protected, for all possible voltage dips and durations. Removing the battery for a few seconds, then replacing it should give a clean RTC reset.