STM32F779II Flash not erased

I'm trying to erase some sectors from the flash, so that I can write to them.

I'm using a setup of dual bank mode.

For the setup I'm doing this:

HAL_FLASH_Unlock();

  HAL_FLASH_OB_Unlock();

  FLASH->OPTCR &= ~FLASH_OPTCR_nDBANK_Msk;

  HAL_FLASH_OB_Lock();

  HAL_FLASH_Lock();

Following it:

I'm doing flash erase by the following function that takes a sector and erase it.

but it fails, and the address of the sectors that are earsed are not 0xFFFFFFF

secbool flash_erase_sectors(const uint8_t *sectors, int len,
                            void (*progress)(int pos, int len)) {
  ensure(flash_unlock_write(), NULL);
  FLASH_EraseInitTypeDef EraseInitStruct;
  FLASH_OBProgramInitTypeDef OBInit;

  EraseInitStruct.TypeErase = FLASH_TYPEERASE_SECTORS;
  EraseInitStruct.VoltageRange = FLASH_VOLTAGE_RANGE_3;
  EraseInitStruct.NbSectors = 1;
  if (progress) {
    progress(0, len);
  }



    __HAL_FLASH_ART_DISABLE();
    __HAL_FLASH_ART_RESET();
    __HAL_FLASH_ART_ENABLE();


    /* Allow Access to option bytes sector */
    HAL_FLASH_OB_Unlock();
    /* Get the Dual bank configuration status */
    HAL_FLASHEx_OBGetConfig(&OBInit);
    /* Allow Access to option bytes sector */
    HAL_FLASH_OB_Lock();


  if((OBInit.USERConfig & OB_NDBANK_SINGLE_BANK) == OB_NDBANK_DUAL_BANK){
        /*  We don't support the DUAL BANK MODE for now, so return error */
    }


  /* Unlock the Flash to enable the flash control register access *************/
   ensure(flash_unlock_write(), NULL);
    /* Allow Access to option bytes sector */ 
  HAL_FLASH_OB_Unlock();
  for (int i = 0; i < len; i++) {
    EraseInitStruct.Sector = sectors[i];
    uint32_t SectorError;
    if (HAL_FLASHEx_Erase(&EraseInitStruct, &SectorError) != HAL_OK) {
      ensure(flash_lock_write(), NULL);
      return secfalse;
    }
    // check whether the sector was really deleted (contains only 0xFF)
    const uint32_t addr_start = FLASH_SECTOR_TABLE[sectors[i]],
                   addr_end = FLASH_SECTOR_TABLE[sectors[i] + 1];
    for (uint32_t addr = addr_start; addr < addr_end; addr += 4) {
      if (*((const uint32_t *)addr) != 0xFFFFFFFF) {
        ensure(flash_lock_write(), NULL);
        HAL_FLASH_OB_Lock();
        return secfalse;
      }
    }
    if (progress) {
      progress(i + 1, len);
    }
  }
  HAL_FLASH_OB_Lock();
  ensure(flash_lock_write(), NULL);
  return sectrue;
}