PowerShell: Write-Output / Write-Warning print sequence for PsCustomObject

Question

The following code:

$test = @( [PsCustomObject] @{ PSTypeName  = 'Type'; Field1 = "a"} )
$test2 = @("x")

Write-Output $test

foreach ($x in $test2)
{
    Write-Warning $x
}

gives this output:

WARNING: x
Field1
------
a

Why is the order in which things are printed different from expected - Write-Output being first in the code. Note that this only happens if $test is a PsCustomObject.