I understand the loss of information is in the program (flash) memory, not in the data (E2PROM) memory. Is this true?.

I have used the AT89S53 (similar to an ATS8252 but without data -E2PROM- memory) and I have experienced problems in only one application. In that application some of the P1 port lines of the SPI were connected as inputs. If this is the case, care must be taken not to enter accidentally in the serial downloading mode, while in the power-up sequence. In my application the problem was solved changing some assignations of input-output lines. The Atmel's handbook explains -briefly- this problem.