Creating subfolders with Applescript for Apple Mail Leads to oddly named subfolder. What are potential coding solutions or workarounds?

I'm trying to build an AppleScript against Apple Mail to create a mail subfolder under another parent folder. The mail server is Microsoft exchange. While the script runs successfully, it creates the subfolder named as "ParentFolder/ChildFolder" under ParentFolder.

The script is simple:

on run argv
    if (count of argv) is not 2 then
        error "Usage: osascript create_mail_subfolder.applescript <parent-folder-name> <subfolder-name>"
    end if
    
    set parentFolderName to item 1 of argv
    set subfolderName to item 2 of argv
    
    tell application "Mail"
        tell account "Exchange"
            set NewMailbox to make new mailbox with properties {name:parentFolderName & "/" & subfolderName}
        end tell
    end tell
end run

The script runs but I get a folder called "Customers/testfolder" under "Customers":

% osascript testcreate.applescript "Customers" "testfolder"

I have confirmed that the subfolder I want to create doesn't already exist. The "Customers" folder does exist, but "testfolder" does not.

I have even tried renaming the "Customers/testfolder" folder to just "testfolder" but get a -10006 error, which suggests the name of the folder is read-only.

% osascript testcreate.applescript "Customers" "testfolder"
Customers/testfolder
testcreate.applescript:671:705: execution error: Can’t set name of "Customers/Customers/testfolder" to "Customers/testfolder". (-10006)

I see from this old MacRumors post that I'm not alone. Could anyone share a workaround for this issue or is there another way to create subfolders against Apple Mail?