why does IWDG not freeze in standby mode although IWDG_STOP bit is set to frozen in the fist time?

[NPHUO]

I am using NUCLEO L476RG board to test IWDG is frozen in Standby mode. I did following steps in below

1. set IWDG_STDBY bit in FLASH_OPTR to zero
2. Initialize Iwdg will reset in 5s
3. Start Standby mode by HAL library

the board was reseted​ in 5s, but it was not reseted when i retried in the second or after.

please show me how to frozen IWDG without reset in the fist time?