Problems writing with I2C to registers in Allegro ACS71020
The  ACS71020 supports factory and customer EEPROM space as well as
  volatile registers.  The customer access code must be sent prior to
  writing these customer EEPROM spaces. In addition, the device includes
  a set of free space EEPROM registers that are accessible with or
  without writing the access code.


Th datasheet quotes about access code. Check whether the location you are using is in need of it or not.