This is not a bug.
The File folder entry is a string. To add a php value, you must enclose it in php tags.
If you where to click the lightning bolt and select the UserID session variable, the File name box would look like:
<?php echo $_SESSION['UserID']; ?>
you can then add the rest of the folder location around that:
../images/Users/<?php echo $_SESSION['UserID']; ?>/
this will cause the code to be entered corretcly as:
'UploadFolder' => "../images/Users/".$_SESSION['UserID'] ."/",