My guess is that it may be a limitation of Windows due to it using a case-insensitive file system. If you just attempt to change the case of the name, Windows thinks the new name is the same as the old one. For example of this, open Windows Explorer and browse to a local folder, then try to change the case of a letter in a filename. It won't stick. You have to change the name somehow first.
So it sounds like you've discovered the simple workaround. Rename the file twice, making some arbitrary change to the name the first time, and then undo it the second time. This is very easy to do in an Action group. I wouldn't use the space trick, because if the second renaming fails for some reason you may not notice the change. Add something more obvious, like XXXXX.