Powershell replace from array strange behaviour - powershell

Question

Problem is solved, but I don't understand why :-)
I have a Powershell script that perform replacements inside files (language metadata):
loads a list of replacement from a txt file into an array
gets all xml files from a Start folder
performs all the replacements from the array
performs a replacement on the filename based on the array first entry
saves the resulting files in a End folder
I've been using successfully variations of the exact same script for many years, with the only thing changing being the replacement file name and content... except today when creating another variant. The only change was the content of the substitution file, and suddenly the replacement did not happen anymore in the filename.
Here is the code:
#load the replacements from file
$data = Import-Csv -Path substitutions.txt -Header "Source", "Target", "Safe", "Count" -Delimiter "|"
#load the files to be processed
$xmlfiles = -join ($Startfolder, "*.xml")
$Fileset = Get-ChildItem $xmlfiles -recurse
foreach ($File in $Fileset) {
$NewFileName = ""
$WipFile = Get-Content $File
# set safe replacement flag to nothing
$flag = ""
#perform replacements
foreach ($item in $data) {
if ($WipFile -cmatch $item.Source) {
if ($item.Safe -eq 'yes') {
$WipFile = $WipFile -creplace $item.Source, $item.Target
$item.Count = $item.Count + 1
}
else {
$WipFile = $WipFile -creplace $item.Source, $item.Target
$item.Count = $item.Count + 1
$flag = "TOCHECK "
}
}
}
#replace language code in filename, based on first entry in the substitution list
$NewFileName = -join ($Endfolder, $flag, $file.name -creplace $data.Source[0], $data.Target[0])
Write-Host $NewFileName
#save file with updated content
$WipFile | Set-Content -Encoding Unicode ($File)
#move file to End folder
Move-Item $File $NewFileName
}
The substitution file is formatted as follows:
nl-NL|nl-BE|yes
After testing more, I discovered my new variant was failing if my substitution file had only one line. Add another one, and it works. How come?