R7920 Memory Mapping I/O keep reseting

We have a R7920 that have 2x Xeon Gold 6154 , 768GB RAM (12x6gb) but as soon as we add a GPU to (slot 1 riser 1 or any other slot) the system Memory Mapping I/O setting change to 512GB and limited the memory to 511GB. We try to set it back to 56TB or 12TB but after reboot the system Memory Mapping I/O setting change back to 512GB. We have change other setting to confirm it not the system resetting the BIOS every time and confirmed the other setting remain the same but the Memory Mapping I/O keep resetting to 512GB.