Start-Process using EncodedCommand outputs to Standard Error - powershell

Question

The following script demonstrates an issue I'm experiencing after moving from PowerShell v4 to v5.1. I make extensive use of Start-Process redirecting StandardError to a file, if this file contains data after execution I can read the file to see what the issue was.
However, with v5.1 when the launched PowerShell process starts, it makes use of progress output which is on the error stream, so this is redirected to the error file. In previous versions of PowerShell this did not happen, if I use Command rather than EncodedCommand it works as expected.
I believe this is a bug but it would be useful if someone could confirm or suggest a workaround?
The output to the script below is as follows:
PowerShell v4
No Errors
hello world!
PowerShell v5.1
WARNING:
SourceId Record
1 parent = -1 id = 0 act = Preparing modules for first use. stat = cur = pct = -1 sec = -1 type = Completed
hello world!
hello world!
Demo Script
Note: this will create 2 temporary files, which are both deleted at the end.
$outfile = [System.IO.Path]::GetTempFileName()
$errorfile = [System.IO.Path]::GetTempFileName()
$command = "write-host 'hello world!'"
$encodedCommand = [Convert]::ToBase64String([Text.Encoding]::Unicode.GetBytes($command))
$argumentList = #(
"-NoLogo",
"-NonInteractive",
"-NoProfile",
"-EncodedCommand",
$encodedCommand
)
Start-Process powershell `
-argumentlist $argumentList `
-PassThru `
-wait `
-RedirectStandardOut $outfile `
-NoNewWindow `
-RedirectStandardError $errorfile | Out-Null
if ((Get-Item $errorfile).length -gt 0)
{
Import-Clixml $errorfile | Out-String | Write-Warning
}
else
{
Write-Host "No Errors"
}
Get-Content $outfile
$outfile, $errorfile | Remove-item