Additionally, you can't change the OS profile for an existing VM, and the properties are set correctly as read-only.
Microsoft confirms that this is a limitation in the Azure platform. The effect of this limitation is limited to those who want the properties data to accurately reflect the values that are provided on the portal.
Note Although the computer name field is blank, the VM name that's provided during resource creation is still displayed. Also, the host name is present inside the guest OS and is unchanged after the existing or specialized disk is first configured.