Flash memory erased issue

hello,

Download the app uart example of central, turn off the power,

then turn on the power again after a few days When I tried to do it, nothing happened.

So, after verifying, the flash was deleted as shown below.

- sdk 17.1.0

- nRF52-DK

When re-writing again, the motion is normal... 
I wonder what the problem is.

thank.

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