Azure plugin 2.5.1: sysprepping not returning and packer hanging + powershell scripts not streaming

Overview of the Issue

Issues present in 2.5.1 and not in 2.5.0.

Issues:

• the powershell provisioner which runs my sysprep script won't return and packer hangs indefinitely (while run from Azure pipeline)
• the powershell provisioner running other scripts won't stream the output, but just dump the output at the end of the script

Reproduction Steps

Try and create a Windows 11 (e.g. microsoftwindowsdesktop:windows-11:win11-24h2-avd) with the azure plugin 2.5.1. In the packer configuration add scripts that output with "Write-Output". Outputs are not streamed but are dumped at the end of the script.

More problematic is when a sysprep script is run with the powershell provisioner, the script will effectively run the sysprep, but packer will end without returning from the call.

Plugin and Packer version

Packer and its plugins are distinct binaries.

Packer version: "1.14.1"

Azure plugin: "2.5.1"

Simplified Packer Buildfile

packer {
  required_plugins {
    azure = {
      source  = "github.com/hashicorp/azure"
      version = "2.5.1"
    }
  }
}

source "azure-arm" "golden-image" {
  use_azure_cli_auth                     = true
  communicator                           = "winrm"
  winrm_username                         = local.admin_username
  winrm_timeout                          = "5m"
  winrm_insecure                         = true
  winrm_use_ssl                          = true
  image_offer                            = var.image_offer
  image_publisher                        = var.image_publisher
  image_sku                              = var.image_sku
  location                               = "uksouth"
  os_type                                = "Windows"
  os_disk_size_gb                        = var.config.images_settings[var.image_name].os_disk_size_gb
  vm_size                                = "Standard_B4ms"
  user_assigned_managed_identities       = [var.config.user_assigned_managed_identity_id]
  virtual_network_name                   = var.config.virtual_network_name
  virtual_network_subnet_name            = var.config.virtual_network_subnet_name
  virtual_network_resource_group_name    = var.config.virtual_network_resource_group_name
  private_virtual_network_with_public_ip = var.config.private_virtual_network_with_public_ip


  shared_image_gallery_destination {
    subscription         = var.gallery_subscription_id
    resource_group       = var.gallery_resource_group
    gallery_name         = var.gallery_name
    image_name           = var.image_name
    image_version        = var.image_version
    storage_account_type = "Standard_LRS"

    target_region {
      name = "uksouth"
    }
  }

build {
  sources = ["source.azure-arm.golden-image"]

  # Install all tools
  provisioner "powershell" {
     environment_vars = [ 
     # truncated
]

scripts = concat(
      local.core_image_scripts,
      local.image_specific_scripts
    )

    timeout = var.config.install_tools_timeout

}

provisioner "windows-restart" {
    pause_before          = "120s"
    restart_check_command = "powershell -command \"& {Write-Output 'Machine restarted.'}\""
    restart_timeout       = "60m"
  }

# other provisioners truncated

# sysprepping
provisioner "powershell" {

    environment_vars = [
      "RUN_FROM_PACKER=true"
    ]

    scripts = [
      "${path.root}/../../scripts/prepare-windows-vm.ps1"
    ]

    elevated_user     = local.admin_username
    elevated_password = build.Password
  }

Operating system and Environment details

Runner: linux image on an Azure Managed DevOps Pool

Log Fragments and crash.log files

After 12 hours of running, the pipeline gets cancelled and the last few lines of the output is:

==> azure-arm.golden-image: Provisioning with Powershell...
==> azure-arm.golden-image: Provisioning with powershell script: /mnt/vss/_work/1/src/image-builder/core-components/../../scripts/prepare-windows-vm.ps1
##[error]The Operation will be canceled. The next steps may not contain expected logs.
##[error]The operation was canceled.

The prepare-windows-vm.ps1 runs successfully (I can see the sysprep logs from the azure VM itself), but packer won't continue further.

Expected next steps: saving the image to the gallery.

Start with the powershell provisioner and the referenced scripts/prepare-windows-vm.ps1, reproducing the simplified Azure build on Windows 11 with Write-Output and sysprep. Compare behavior between Azure plugin 2.5.0 and 2.5.1. Done means script output streams during execution, sysprep returns to Packer, and the build proceeds to saving the image to the gallery.