The most likely cause is that you did not enable VDD Monitor.

If the VDD Monitor is disabled, data on the flash memory may be corrupted depending on the contents of the user code.
VDD Monitor must be enabled when erasing/writing to flash memory from user code. Please be sure to read the description "Voltage Supply Maintenance and the Supply Monitor" in the "Flash Memory" chapter of each family's data sheet (reference manual for EFM8).
Normal operation cannot be expected during the period when the supply voltage is not within the guaranteed operating range (during power on/off, momentary power failure), but during this period the program counter shows an abnormal value and an unspecified part of the user code is executed. You may If the user code includes an erase/write to the flash memory, the erase/write will be executed unintentionally, causing data corruption.

Regarding flash data corruption, checkpoints are summarized on the SiliconLabs community site, so please check them as well.