When perform the TI provided  sdl_ex_flash_ecc_test.

Technical Reference Manual documents . When ECC test mode is enabled, the CPU cannot read the data from Flash and instead the CPU gets data from the ECC test mode registers (FDATAH_TEST/FDATAL_TEST). This is because ECC test mode registers (FDATAH_TEST, FDATAL_TEST, FECC_TEST) are multiplexed with data from the Flash.

From my understanding that the real flash address shall also have the same data 0xFEDCBA0987654321ULL,  but in the debug, it shows not.

How to understand the  ECC test mode registers (FDATAH_TEST, FDATAL_TEST, FECC_TEST) are multiplexed with data from the Flash.