When you tried to update the SA0 location, which bootloader executed the erase/write commands, at SA1 or SA2?
I expect that the bootloader at SA1 issued the erase/write commands to update SA3 and SA0.
The dual operation flash memory of MB95770 consists of two banks and cannot be erased/programmed and read (instruction-fetched) at the same time in bank on the same side.
SA0 and SA1 are located in the same bank, while SA2 and SA3 are located in another bank.
So, the bootloader at SA1 can erase/write the SA3, but cannot do the SA0.
The bootloader at SA2 can erase/write the SA0, but cannot do the SA3.
Please try erase/write the SA0 from the bootloader at SA2.