Powershell – Create a new VHD and mount it without DiskManagement window popup with Powershell

How can I create and mount a new VHDX volume without a drive letter assigned?
On some servers I get the behaviour described below, on others it works like it should.
So there must be a system setting which affect this behaviour besides AutoMount.

I use the following code to create and mount a VHDX volume with Powershell

New-VHD -path $vhdpath -SizeBytes $vhdSize -Dynamic | `
Mount-VHD -NoDriveLetter -Passthru

This creates the VHDX file and mounts ist into the system.
Then the DiskManagement window popus up instantly, asking me to initialize the new volume.
But I want to initialize, partition and format the volume using scripting.

I found hints to disable the windows AutoMount feature, but this did not help.

Also the system assigns a random drive letter to the volume when I manuall abort the initializing dialog and proceed with the following commands:

$d = InitializeDisk -Number 4 -PartitionStyle GPT
$p = New-Partition -DiskNumber $d.Number -UseMaximumSize -AssignDriveLetter:$False
$v = Format-Volume -Partition $p -FileSystem NTFS -Confirm:$false
Add-PartitionAccessPath -DiskNumber $d.Number -PartitionNumber $p.PartitionNumber -AccessPath "C:\mount"

Then the partition is created and mounted into C:\mount.
But the system also assigns a drive letter. I do not want this.

System: Windows Server 2012 R2 Standard
Shell: Powershell 4.0