so, you think that the problem is that chip has not programmed with correct way? Does your program use API/ISP (do not confuse with SPI).
In fact, it may be problem: T89C51RD2 has bootloader inside flash area and so you need to re-program it after full chip erase; opposite it, AT89C51RD2 has bootloader inside separate ROM and may not be reprogrammed. So, if your program uses bootloader calls (API) for T89C51RD2 then these routines must be rewritten with mapping bootloader ROM into code space (via AUXR1/ENBOOT bit).

Regards,
Oleg