First, are you sure you've got the industrial or better grade PIC ?

It's interesting that only the first byte doesn't get written. This suggests
that maybe the PIC and crystal are OK

Are you using SLEEP and WDT at all ? 32kHz ?

WDT can be 25% faster at -18C than at 25C and I wonder if there's
some problem allowing enough time for the PIC's oscillator to wake
up for the first time. A problem that doesn't seem isn't there at higher
temperatures, although you could be on the ragged edge and not
know it

The 32kHz LP can take quite a while to stabilise, even at room temp

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