You shouldn't rely on FlashMagic saying OK. FlashMagic does none of the programming, the internal bootloader does, which is only a piece of software. Although it might use a different reading mechanism than simply using MOVC, it still might not be a fully valid read test (on the "H" it does read using the built-in ISP hardware - but as I said I have no non-H P89C51RD2).

My idea of what might happen is that the flash gets undererased or underprogrammed at certain parts, and that the code fetch has a different timing than the MOVC/ISP read, which might reveal the undererasure/underprogramming. I can't confirm this as am not a Philips insider and don't know the details of their design, I even don't have a piece of P89C51RD2nonH to try... But for you to try to enter 0MHz into FlashMagic takes 10 seconds, so why won't you try?

Jan Waclawek