This question is about when the RTC transits to VBAT Power State (Fig. 29) after VCC power supply is supplied and VBAT is also supplied. Even in the VBAT Power State, if the voltage when the VBAT voltage drops is V_BATRST < VBAT, is the BL bit of the status register set correctly?
The BL bit is intended to monitor VBAT while in VCC powered state. VBAT is intended as backup power to keep time when system power is removed.