Packer null builder hangs on WinRM even though Test-WSMan succeeds

I have a VM setup up on Hetzner running Windows Server 2022 that I want to apply the Github Action Runner template to. I have modified the template to use the null builder with WinRM communication. I have manually performed PS remoting (e.g. Enter-PSSesion) to the target server using the credentials, and Test-WSMan -ComputerName <server-ip> succeds.

    "builders": [
            "type": "null",
            "communicator": "winrm",
            "winrm_username": "my_user",
            "winrm_password": "my_password",
            "winrm_host": "",
            "winrm_insecure": true,
            "winrm_use_ssl": false

I simply end up with this:

packer build -debug .\windows-2022.json
Debug mode enabled. Builds will not be parallelized.
null: output will be in this color.

==> null: Using WinRM communicator to connect:
==> null: Waiting for WinRM to become available...

Any ideas about what might be causing this? Is packer using a different mechanism for connecting?