Inconsistent binding when piping DirectoryInfo objects

I've often used a construct similar to this (using aliases for brevity):

gci -ad | %{$_ | gci}

which works fine. But when trying to help another user on this forum, I found that the following doesn't work:

gci -ad | %{$_.Parent | gci}

throws the following error for each iteration:

gci : Cannot find path 'C:\Users\keith\Documents\Documents' because it does not exist.
At line:1 char:25
+ gci -ad | %{$_.Parent | gci}
+                         ~~~
    + CategoryInfo          : ObjectNotFound: (C:\Users\keith\Documents\Documents:Stri
   ng) [Get-ChildItem], ItemNotFoundException
    + FullyQualifiedErrorId : PathNotFound,Microsoft.PowerShell.Commands.GetChildItemC
   ommand

even though:

gci -ad | %{$_.GetType() -eq $_.Parent.GetType()}

produces a scrennfull of True.

I'm pretty sure it has something to do with parameter binding, but would like to understand the apparent inconsistancy....

Easiest way to see what's happening is to emulate the binding with a test function, but the issue is, as you might know in .NET Framework when a DirectoryInfo instance is coerced into a string the result will be it's .Name property value as opposed to .NET where the coercion results in the .FullName property.

Since the object returned by calling the .Parent property does not have the ETS Property .PSPath, the input object will be bound the the Path Parameter and coerced to a string.

Assuming you have both versions of PowerShell, you can try the following to see the difference:

function Test-Binding {
    [CmdletBinding(DefaultParameterSetName='Items')]
    param(
        [Parameter(ParameterSetName='Items', Position=0, ValueFromPipeline=$true, ValueFromPipelineByPropertyName=$true)]
        [string[]]
        ${Path},

        [Parameter(ParameterSetName='LiteralItems', Mandatory=$true, ValueFromPipelineByPropertyName=$true)]
        [Alias('PSPath')]
        [string[]]
        ${LiteralPath}
    )

    process {
        $PSBoundParameters
        $PSBoundParameters.Clear()
    }
}

(Get-Item .), (Get-Item .).Parent | Test-Binding