There are 2 reasons may cause NV RAM data corrupted or lost,
1. If the t F (V CC Slew Rate Power-Down) is too short (ie, < 300usec) then the comparator that looks at V CC versus battery voltage may not have enough time to determine it needs to switch to battery power. This means V CC will remain as the supply to the RAM, and if the V CC drops too low then the voltage will not be sufficient to maintain the charge on the RAM and data will be lost/corrupted.
2. Another cause could be any negative voltages on any pin that go below 0V. Negative undershoots on any pin can affect the substrate reference for the RAM cells and cause data corruption.