Terraform - Dynamic Security Rules not working in Azure

I am unable to see the dynamic rules getting created in Azure Network Security Group (NSG). My template looks ok from validation checks. However, when i run the terraform apply the NSG creates without the custom rules i put under dynamic rules section. Since this is part of a module i have to use some logic for iteration.

Below is my code from module.tf file

resource "azurerm_network_security_group" "nsg" {
for_each            = var.subnet_prefix
name                = lower("nsg_${each.key}_in")
resource_group_name = var.vnet_resourcegroup
location            = var.location
dynamic "security_rule" {
for_each = lookup (each.value, "nsg_rules", [])
content {
name                       = lookup(security_rule.value,"name",null)
description             =  lookup(security_rule.value, "description", null)
priority                   = lookup(security_rule.value,"priority",null)    
direction                  = lookup(security_rule.value,"direction",null)
access                     = lookup(security_rule.value,"access",null)
protocol                   = lookup(security_rule.value,"protocol",null)
source_port_range          = "*"
destination_port_range     = lookup(security_rule.value,"destination_port_range",null)
source_address_prefix      = lookup(security_rule.value,"source_address_prefix",null)
destination_address_prefix = lookup(security_rule.value,"destination_address_prefix",null)
}
}
}

The values will be coming from a terraform.tfvars file , which i have declared as under. For compliance i have removed the original values with random text.

subnet_prefix = {
sa0 = {
prefix = "dc-subnet"
address_space = ["XXXXXX"]
nsg_name = "nsg0"
nsg_rules = [
{
        name                         = "rule1"
        description                  = "XXXXXX"
        priority                     = 220
        direction                    = "Outbound"
        access                       = "Allow"
        protocol                     = "*"
        source_port_range            = "*"
        source_port_ranges           = null
        destination_port_range       = *
        destination_port_ranges      = null
        source_address_prefix        = "*"
        source_address_prefixes      = null
        destination_address_prefix   = "X.X.X.X/26"
        destination_address_prefixes = null
        source_application_security_group_names      = []
        destination_application_security_group_names = []
      }
]
}
}