Maybe some variable changes value of some new special register in AT89S52?
It cannot be truth as long as you use memory with correct way:
[0x00...0x7F] - for both direct and indirect access;
[0x80...0xFF] - for either indirect (@R0/@R1) access or stack area only.
Indeed, if you somehow utilize memory [0x80...0xFF] with direct access then you should know that it is not a user memory but SFRs instead. But why do you write into SFRs which are not defined for C52 then?
Good days!