I have a similar problem. I am attempting to assign a value to a variable and then execute the powershell script using that variable. The SQL agent job step is CmdExec and it executes or completes successfully but does not update the underlying table like it does in using the powershell window alone. Here is the code from the job step:

powershell.exe "$dt = D:\PSscripts\Invoke-Sqlcmd2.ps1 -ServerInstance 'JAXSQLMON01' -Database DBACentral -Query 'Select server_name from server_instance' | foreach-object {D:\PSscripts\Invoke-Sqlcmd2.ps1 -ServerInstance $_.server_name -Database master -InputFile ./D:\PSscripts\get-dbspace.sql -As 'DataRow'}"

powershell.exe "D:\PSscripts\Write-DataTable.ps1 -ServerInstance 'JAXSQLMON01' -Database 'DBACentral' -TableName 'db_Space' -Data $dt"

Any help would be greatly appreciated. When I run this using regular powershell window it creates the variable and then populates the table with the file sizes which I am trying to collect.