The datasheet states there is no need to erase the cells before reprogramming.
I have not seen such flash before either.
Maybe its some kind of eeprom.

Here some quotes from the datasheet: (p.93)
Flash Programming and Erasure:
The 16K or 32K bytes Flash is programmed by bytes or by pages of 128 bytes. It is not
necessary to erase a byte or a page before programming. The programming of a byte or
a page includes a self erase before programming.

Thomas