The issue is that you are reading the values from your CSV and directly binding into the TreeList field but the items you are binding don't exist in Sitecore.
If you will see the raw value of the TreeList Field it's a combination of Sitecore Item IDs with a pipe(|) separator. So in your case, those items do not exist in the Sitecore database and that's why they are showing like this.
So the code will be something like this.
$services = $row."Services".Split(",");
if($itemsGuidFromYourTreeListField -ne $null)
foreach($service in $services)
# Get item by it's Path
$currentItem = Get-Item -Path $service.ItemPath
# Write logic to append the ID of $currentItem in pipe separated ID's
$item = Get-Item -Path "master:" -ID "<Your Item ID>"
$item.Fields["YourTreeListField"].Value = "<pipe separated ID field>";
And if you have the ID's of items that exist in your Sitecore database from your CSV then you don't need to use the method
Get-Item to find those. In this case, you can loop through to create a string like this.
And directly use begin and end edit to update your field.
Hope this helps.