azurerm - Terraform not behaving as expected

I'm trying to create a Terraform project to create everything I need in an Azure subscription, so resource groups, vnets, subnets and VM's.

However when I've run this once and try again, it states that it cannot delete a subnet that is in use. I haven't changed anything about the subnet or the VM connected to it.

Error: creating/updating Virtual Network: (Name "" / Resource Group ""): network.VirtualNetworksClient#CreateOrUpdate: Failure sending request: StatusCode=400 -- Original Error: Code="InUseSubnetCannotBeDeleted" Message="Subnet build-agent is in use by /subscriptions/mysub/resourceGroups/myrg/providers/Microsoft.Network/networkInterfaces/mynic/ipConfigurations/internal and cannot be deleted. In order to delete the subnet, delete all the resources within the subnet. See aka.ms/deletesubnet." Details=[]

terraform {
  required_version = ">= 1.1.0"
  backend "azurerm" {
  }
  required_providers {
    azurerm = {
    version = "=3.5.0"
      source = "hashicorp/azurerm" # https://registry.terraform.io/providers/hashicorp/azurerm/latest
    }
  }
}

# Configure the Microsoft Azure Provider
provider "azurerm" {
  features {}
}

locals {
    name_suffix = "<mysuffix>"
}

resource "azurerm_resource_group" "rg-infra" {
  name     = "rg-${local.name_suffix}"
  location = "UK South"
}

resource "azurerm_virtual_network" "vnet-mgmt" {
  name                = "vnet-${local.name_suffix}"
  location            = azurerm_resource_group.rg-infra.location
  resource_group_name = azurerm_resource_group.rg-infra.name
  address_space       = ["<myiprange>"]

  subnet {
    name           = "virtual-machines"
    address_prefix = "<myiprange>"
  }

  subnet {
    name           = "databases"
    address_prefix = "<myiprange>"
  }
}

data "azurerm_virtual_network" "network" {
  name                = "vnet-${local.name_suffix}"
  resource_group_name = azurerm_resource_group.rg-infra.name
}

resource "azurerm_subnet" "sb-ansible" {
  name                 = "build-agent"
  resource_group_name  = azurerm_resource_group.rg-infra.name
  virtual_network_name = data.azurerm_virtual_network.network.name
  address_prefixes     = ["<myiprange>"]
  depends_on           = [azurerm_virtual_network.vnet-mgmt]

}

data "azurerm_subnet" "prd-subnet" {
    name = "build-agent"
    virtual_network_name = data.azurerm_virtual_network.network.name
    resource_group_name = azurerm_resource_group.rg-infra.name
    depends_on          = [azurerm_subnet.sb-ansible]
}


resource "azurerm_network_interface" "ni-ansible" {
  name                = "nic-ansible-${local.name_suffix}"
  location            = azurerm_resource_group.rg-infra.location
  resource_group_name = azurerm_resource_group.rg-infra.name

  ip_configuration {
    name                          = "internal"
    subnet_id                     = data.azurerm_subnet.prd-subnet.id
    private_ip_address_allocation = "Dynamic"
  }

  lifecycle {
    ignore_changes = ["ip_configuration"]
  }

  depends_on           = [azurerm_subnet.sb-ansible]
}

resource "azurerm_linux_virtual_machine" "ansible-vm" {
  name                = "ansible-build-agent"
  resource_group_name = azurerm_resource_group.rg-infra.name
  location            = azurerm_resource_group.rg-infra.location
  size                = "Standard_D2as_v4"
  admin_username      = "myadminuser"
  network_interface_ids = [
    azurerm_network_interface.ni-ansible.id,
  ]

  admin_ssh_key {
    username   = "myadminuser"
    public_key = ""
  }

  os_disk {
    caching              = "ReadWrite"
    storage_account_type = "Standard_LRS"
  }

  source_image_reference {
    publisher = "Canonical"
    offer     = "UbuntuServer"
    sku       = "18.04-LTS"
    version   = "latest"
  }

  lifecycle {
    ignore_changes = ["source_image_reference"]
  }

  depends_on           = [azurerm_network_interface.ni-ansible]
}

Any help on why it's behaving like this, or a workaround would be greatly appreciated!

Many thanks

Turns out you can't mix nested subnets in the vnet block with an explicitly defined azurerm_subnet

So

resource "azurerm_subnet" "example" {

and

resource "azurerm_virtual_network" "example" {
name                = "example-network"
location            = azurerm_resource_group.example.location
resource_group_name = azurerm_resource_group.example.name
address_space       = ["10.0.0.0/16"]
dns_servers         = ["10.0.0.4", "10.0.0.5"]

subnet {
  name             = "subnet1"
  address_prefixes = ["10.0.1.0/24"]
}

cannot coexist. See note here: https://registry.terraform.io/providers/hashicorp/azurerm/latest/docs/resources/subnet