Azure Virtual Machine Scale Set is not returning the publicIPPrefixID after a SKU update

To reproduce:
1. Create a VMSS with a publicIPPrefixID assigned by using the following tf config
```
resource "azurerm_linux_virtual_machine_scale_set" "linux_nvavss" {
name = "test"
location = azurerm_resource_group.main.location
resource_group_name = azurerm_resource_group.main.name
computer_name_prefix = "dapzhang"
admin_username = "dapzhang"
admin_password = "Passw0rd123!"
disable_password_authentication = false
instances = 2

# automatic rolling upgrade
upgrade_mode = "Manual"
sku = "Standard_F2"

zones = []

source_image_reference {
publisher = "Canonical"
offer = "UbuntuServer"
sku = "16.04-LTS"
version = "latest"
}

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

network_interface {
name = "eth0"
primary = true
enable_accelerated_networking = true
enable_ip_forwarding = false
ip_configuration {
name = "ipconfig1"
primary = true
subnet_id = azurerm_subnet.front.id
load_balancer_backend_address_pool_ids = [azurerm_lb_backend_address_pool.test.id]
public_ip_address {
name = "instancePublicIP"
idle_timeout_in_minutes = 30
domain_name_label = "testdomain"
public_ip_prefix_id = azurerm_public_ip_prefix.test.id
}
}
}
}
```
1. Update the VMSS sku with the following request body:
```
"properties": {
"upgradePolicy": {
"mode": "Manual"
},
"virtualMachineProfile": {
"networkProfile": {
"networkInterfaceConfigurations": [
{
"name": "eth0",
"properties": {
"primary": true,
"enableAcceleratedNetworking": true,
"dnsSettings": {
"dnsServers": []
},
"ipConfigurations": [
{
"name": "ipconfig1",
"properties": {
"subnet": {
"id": "/subscriptions/85b3dbca-0000-0000-0000-67a141095a76/resourceGroups/test/providers/Microsoft.Network/virtualNetworks/network/subnets/front"
},
"primary": true,
"publicIPAddressConfiguration": {
"name": "instancePublicIP",
"properties": {
"idleTimeoutInMinutes": 30,
"dnsSettings": {
"domainNameLabel": "testdomain"
}
}
},
"privateIPAddressVersion": "IPv4",
"applicationGatewayBackendAddressPools": [],
"applicationSecurityGroups": [],
"loadBalancerBackendAddressPools": [
{
"id": "/subscriptions/85b3dbca-0000-0000-0000-67a141095a76/resourceGroups/test/providers/Microsoft.Network/loadBalancers/acctestlb-001/backendAddressPools/test"
}
],
"loadBalancerInboundNatPools": []
}
}
],
"enableIPForwarding": false
}
}
]
}
}
},
"sku": {
"name": "Standard_D3_v2",
"tier": "Standard",
"capacity": 2
}
}
```
1. Get the updated VMSS and found publicIPPrefixID is empty.

Here are two assumptions:
1. The PATCH request erased the public ip prefix
1. The PATCH request did not change the public ip prefix, but it just did not returned after the PATCH is done