nRF52810 program run failure

  Hi, I am from China. My company made a BLE module with chip nRF52810. Recently , I coded a program for it by SDK12, based on ble_app_uart example. I modified the corresponding configuration such as FLASH&RAM size so that it can run on the nRF52810 Soc.  I have made it that program run well in the module of nRF52810, than we provided some modules samples to our customer that they would use the modules to cooperate with their MCU. They said ,program worked well.

But after few day, the issues caused. They sent back 3 modules to us. they said , these 3 module doesn't work any more. and the rest of others modules have no this problem. I try to read the FLASH of chips by nrfjprog.exe,  I found, the RBP (readback protection) of one of the modules had been removed, and ALL FLASH IS 0xFF(mean that it is empty).   the RBP of the other 2 pcs are still valid. but they don't work when I  powered for them. Than I try to download a new FW to one modules which has valid RBP, it can run normally. 

  I don't have any idea why this program happened. Our customer said they didn't do anything with a JLINK to the modules. 

  I still remember that once our BLE module made with chip nRF52832 had a similar problem . Do you have any idea with that?  Please give me some advices.