Check progress of database restore running asynchronously
I don't see anything obvious about why this command won't be showing up like any other restore command, unless it is busy doing something other than RESTORE DATABASE. 

I suggest identifying the session_id PowerShell is using (you should be able to see in sys.dm_exec_sessions or sys.dm_exec_connections), or you could change the script to obtain it and print it to the console. Then change your filter to:

SELECT command, status, percent_complete, 
  wait_type, last_wait_type, wait_resource,
  blocking_session_id
FROM sys.dm_exec_requests
WHERE session_id = &lt;x&gt;;


This will help you verify:


if the command is actually a RESTORE or if Powershell is actually doing something else first
current status
percent complete, if that is available (it isn't always)
what it's waiting on (and if you run it repeatedly, if this is changing)
if your restore is actually being blocked by anything