Fsstorage randomly erasing

Our team has been using Fstorage for a long time to store customer data in the device for customization. At random times, a very few units get erased completely when Fstorage is executed. Similar to processing the erase, but not the write. All the registers are FF's

When the device is reprogrammed, it functions normally and to date, has never reoccurred on the same unit.  

Can you offer any insight as to what is happening? The device works on a coin cell battery and happens even if the battery is new. Since it is a very rare occurrence, we cannot duplicate it readily to try and debug. 

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