A7A account missing after flash the os to emmc

after system was flashing from usbdrive to emmc by a7a,i power off the board and take the tfcard out of slot ,then power this board , i found there exist no default account “radxa“and “rock“exist in the new os from emmc so i can not log in
i reboot this board for lots of times and then two default account appears,:grinning_face_with_smiling_eyes: :rofl:

really do not know what’s wrong