We have configured LPM 3.5 with wakeup source as RTC overflow. We check the source of wakeup by reading the SYSRSTIV location which returns the value 0x08(i.e wake up from LPM3.5). It is working fine in several thousand boards. In one of the microcontroller when RTC overflows and the device wakes up from LPM 3.5 , SYSRSTIV register reads the value 0x02(i.e Brownout Reset). This is happening consistently in one particular microcontroller.
Note: We have replaced this microcontroller in different boards which were working fine, we have found that problem is consistent as mentioned above.