Foreach-Object creates CliXML

[fug4zi]

hope someone can me explain why foreach-object creates an clixml.

I use often this code:

$mails = @("[email protected]")
$mails | ForEach-Object {
    $params = @{
        "invitedUserEmailAddress" = $_ #Not Working
        "SendInvitationMessage"   = $true
    }
    Invoke-MgGraphRequest -Uri "https://graph.microsoft.com/v1.0/invitations" -Method POST -Body $params -debug
}

and the debug shows me this:

DEBUG: POST /v1.0/invitations HTTP/1.1
HTTP: graph.microsoft.com
User-Agent: Mozilla/5.0 (Windows NT 10.0; Microsoft Windows 10.0.19045; de-DE) PowerShell/5.1.19041.5486
Invoke-MgGraphRequest
Content-Length: 187
Content-Type: application/json

{"SendInvitationMessage":true,"invitedUserEmailAddress":{"CliXml":"<Objs Version=\"1.1.0.1\"
xmlns=\"http://schemas.microsoft.com/powershell/2004/04\">\r\n  <S>[email protected]</S>\r\n</Objs>"}}

I can fix this by using $_.toString() or $_.trim() Why is this behavior?