I dont use 89S8252 but some general ideas (have some experience with AVR micros):
- HOW do you detect the powerdown? Is it the VCC 5V being lowered for say 10% to 4.5V? If so, check the documentation of the 89S8252 what is the minimum EEPROM programming voltage. It is possible that the voltage during data saving comes to such low level that the normal program execution goes wild. And what is more important, the voltage charge pump is operating thus providing high internal voltage for EEPROM programming. Therefore it is possible that the FLASH SPI download firmware gets executed somehow and accidentaly modifies the FLASH content - note that the high voltage is already present in the chip.
- I suggest you to use different types of powerdown detection:
1. Count the 50Hz pulses if a standard mains power supply is used. When they dissapear you enter the powerdown routine to save the data. Note that the voltage is high enough at this time.
2. Use a standard voltage comparator using an OP amplifier to compare input voltage against the reference. The input voltage is the DC voltage at the input pin of the voltage stabilizer, the reference voltage can be the output voltage of the voltage stabilizer (+5V). Therefore you get the information that the input voltage has been lowered sometime before than using your MAX691. Your output voltage is still 5V when you get the powerdown signal from the comparator.

My suggestion:
- use a voltage comparator for powerdown detection
- use a voltage monitor MAX691 for a 89S8252 RESET when VCC voltage too low to prevent unintentional writing to EEPROM or even FLASH code memory.