PowerShell New-WebServiceProxy doesn't fetch all methods

Question

Im coding a PS script to check the state of a windows service and want to call a specific web service method in certain cases:

$wsdl = "https://webservice-page/wsdl.php?user"
$webserviceUser = "user"
$webservicePassword = "password"
$userId = 1337

$credential = [System.Management.Automation.PSCredential]::new($webserviceUser, (ConvertTo-SecureString $webservicePassword -AsPlainText -Force))

$serviceName = "defragsvc"

$service = Get-Service | Where-Object { $_.Name -eq $serviceName }

write-host "Service Status is" $service.status

 if ($service.status -eq "Stopped")
 {
    $warning = "Service inactive - user with ID " + $userId + " gets notified"
    Write-Warning $warning

    $client = New-WebServiceProxy -Uri $wsdl -Credential $credential

    $output = $client.authenticate($wsdl, $webserviceUser, $webservicePassword)

    echo $output
 }

 if ($service.status -eq "Running")
 {
    echo "OK"
 }

When i fire the script PS throws an error saying the client contains no "authenticte" method, and in fact the IntelliSense doesn't show me all of the existing methods, only a few and they seem randomly selected because i can't see any technical difference in the definition.

Also for every shown method, the IntelliSense duplicates them with "Begin" and "End" at the beginning (see picture)

Soo.. I am very much confused by now.. anyone ever had these kind of problem? Or maybe provide a workaround idea.

I can confirm that the webservices do work fine using PHP SoapClient and __soapCall methods. These scripts are used every day, almost every minute.