STM32F4 SRAM contents retain even after Power reset and standby - stm32f4

Question

I am learning a tutorial about Backup SRAM in STM32f4 controller, in which the microcontroller goes to stand by mode and uses external wake up the pin as wake up source. If backup voltage regulator is not enabled, contents of backup SRAM should be gone, but in my case contents retain. I have even tried reconnecting USB (only power source, no battery supply), and checked the contents of the backup SRAM location through a debugger. At first, it is garbage, as I step through the code, once the backup SRAM clock gets enabled, the location shows "qwerty" characters stored.
Why is this happening?
int main(void)
{
uint32_t * pBackupSRAMbase=0;
char write_buf[] = "qwert";
HAL_Init();
GPIO_Init();
SystemClock_Config_HSE(SYS_CLOCK_FREQ_50_MHZ);
UART2_Init();
//1. Turn on the clock in RCC register for backup sram
__HAL_RCC_BKPSRAM_CLK_ENABLE();
//2. Enable Write access to the backup domain
__HAL_RCC_PWR_CLK_ENABLE();
HAL_PWR_EnableBkUpAccess();
HAL_PWREx_DisableBkUpReg();
pBackupSRAMbase = (uint32_t*)BKPSRAM_BASE;
//Enable clock for PWR Controller block
//__HAL_RCC_PWR_CLK_ENABLE();
if(__HAL_PWR_GET_FLAG(PWR_FLAG_SB) != RESET)
{
__HAL_PWR_CLEAR_FLAG(PWR_FLAG_SB);
__HAL_PWR_CLEAR_FLAG(PWR_FLAG_WU);
printmsg("woke up from the standby mode\r\n");
uint8_t data = (uint8_t)*pBackupSRAMbase;
if(data != 'q')
{
printmsg("Backup SRAM data is lost\r\n");
}
else
{
printmsg("Backup SRAM data is safe \r\n");
}
}else
{
for(uint32_t i =0 ; i < strlen(write_buf)+1 ; i++)
{
*(pBackupSRAMbase+i) = write_buf[i];
}
}
printmsg("Press the user button to enter standby mode\r\n");
while(HAL_GPIO_ReadPin(GPIOC,GPIO_PIN_13) != GPIO_PIN_RESET);
//when user pushes the user button, it comes here
printmsg("Going to Standby mode\r\n");
//Enable the wakeup pin 1 in pwr_csr register
HAL_PWR_EnableWakeUpPin(PWR_WAKEUP_PIN1);
//Enable backup voltage reg.
//HAL_PWREx_EnableBkUpReg();
HAL_PWR_EnterSTANDBYMode();
while(1);
return 0;
}
From the tutorial, I am using, after standby contents should be gone, if backup regulator is not enabled. Also after a power reset it should be gone is no external source is connected. But in both cases contents retain
After reconnecting USB
After enabling backup SRAM clock
IDE: Open STM32 workbench
controller : STM32F466RE
Board: Nucleo