Problem with STM32H7 erasing FLASH bank 1 while running from bank 2

I want to use the FLASH bank-swap feature of the STM32H753 for custom firmware updates.

I have no problem erasing and programming FLASH bank 2 when running from bank 1 but I get a double bus fault when attepting to erase a sector in bank 1 when running from bank 2.

I check SWAP_BANK in FLASH_OPTCR before erasing to determine which bank is currently being run from. I've double checked with STM32Programmer to make sure I'm running from bank 2 and have single stepped in CubeIDE to verify I'm attempting to erase sector 0, bank 1.

Has any one else come across this ?

Thanks

Dave