I haven’t seen this problem. Did you type it in wrong first, then try to change it? Applescript hasn’t been letting you do this for some time (maybe forever). You can either put pipes around the word, or delete the “V” then close Xcode (not just your app), reopen then type in correctly. That should fix it.
Indeed forever. Each instance of AppleScript keeps an internal table of variable names, and once something is added, it’s format is set. I believe it’s something to do with actually using all-lowercase values under the hood.