Powershell: Using .Split on Filename functioning differently based on filename capitalization

I have a bunch of image files that accidentally got renamed with a “doubled” file extension (e.g. IMG_1469.jpg.jpg). I wrote a script to remove the extra file extension, but for some reason, it functions differently based on whether the file extension is in all caps or all lowercase. It functions as intended if the extension is lowercase.

Given two files in the same folder with the following filenames:

$FileA = IMG_1468.JPG.JPG
$FileB = IMG_1469.jpg.jpg

$file = Get-ChildItem -Path "\UNCPathToFile" | Where-Object -Property Name -EQ $FileA
If (($file.BaseName).Length -GT 3) {
        $extCheck = ($file.BaseName).Substring(($file.BaseName).Length - 4,4)
        Write-Host "extCheck =" $extCheck
        If ($file.Extension -EQ $extCheck) {
            $oldName = $file.Name
            Write-Host "oldName =" $oldName
            Write-Host "File.Extension =" $file.Extension
            $newName = ($oldName).Split($extCheck)[0]+$file.Extension
            Write-Host "newName =" $newName
            #$file | Rename-Item -NewName { $newName }
        }
}

Output using $FileA:

extCheck = .JPG
oldName = IMG_1468.JPG.JPG
File.Extension = .JPG
newName = IM.JPG

Output using $FileB:

extCheck = .jpg
oldName = IMG_1469.jpg.jpg
File.Extension = .jpg
newName = IMG_1469.jpg

Anyone have any idea why .Split is removing everything from the base name after “IM” instead of just removing .JPG like it does when the extra extension is .jpg?