Select-object then group-object give hashtable that I can't access by key

Question

If I have a list of objects

$o1 = New-Object –TypeName PSObject –Property @{"MyKey"= "dog"; "MyValue"="Steve"}
$o2 = New-Object –TypeName PSObject –Property @{"MyKey"= "dog"; "MyValue"="Frank"}
$o3 = New-Object –TypeName PSObject –Property @{"MyKey"= "fish"; "MyValue"="Frank"}

$inputs = ($o1, $o2, $o3)

And a function that groups them into a table and then displays them

Function Report
{
    param($records)
    $hashtable = $records | Group-object -AsHashTable -Property "MyKey" 
    Write-host "Format table:"
    $hashtable | format-table
    Write-host "Contains key:"
    $hashtable.Keys | %{ "Key '$_' found: " + $hashtable.ContainsKey($_) }
}

So when I run this over my input list

Report($inputs)

I get

Name                           Value                                                                                                                             
----                           -----                                                                                                                             
dog                            {@{MyKey=dog; MyValue=Steve}, @{MyKey=dog; MyValue=Frank}}                                                                        
fish                           {@{MyKey=fish; MyValue=Frank}}                                                                                                    


Contains key:
Key 'dog' found: True
Key 'fish' found: True

As I would expect.

I can now run my list through select-object

Report($inputs | select-object -Property MyKey, MyValue )

and get the same output from Report as above.

However if I use an expression for the MyKey value:

Report( $inputs | select-object -Property `
    @{ Label = "MyKey"; Expression = {$_.MyKey} },
    MyValue )

I get a grouped table that looks the same, but doesn't allow me to access values by key:

Name                           Value                                                                                                                             
----                           -----                                                                                                                             
dog                            {@{MyKey=dog; MyValue=Steve}, @{MyKey=dog; MyValue=Frank}}                                                                        
fish                           {@{MyKey=fish; MyValue=Frank}}                                                                                                    


Contains key:
Key 'dog' found: False
Key 'fish' found: False

Why can't I access this hashtable by key?

---

Note that this question looks a lot like this one but that solution did not work here. That is:

Add-Type -TypeDefinition @'
    public static class Helper {
        public static object IndexHashtableByPSObject(System.Collections.IDictionary table,object[] key) {
            return table[key[0]];
        }
    }
'@

Function Report
{
    param($records)
    $hashtable = $records | Group-object -AsHashTable -Property "MyKey" 
    $hashtable.Keys | %{ "Get using helper:" + [Helper]::IndexHashtableByPSObject($hashtable,$_) }
    $hashtable.Keys | %{ "Get using reflection:" + [Collections.IDictionary].InvokeMember('','GetProperty',$null,$hashtable,$_) }
}

returns

Get using helper:
Get using helper:
Get using reflection:
Get using reflection:

am I implementing those solutions wrong?