Wow - I did the same, and would not have spotted that!
I was looking at the hyphen, in case that had been converted to a proper hyphen (there are half a dozen different types of hyphen in Unicode, that I know about) rather than being an ASCII minus '-'. Didn't think of looking at the "x", which is centred vertically when it's a multiplication symbol.
After renaming the file, the banner image appeared almost immediately. Perhaps WP.org isn't polling the plugins any more, but just queues up changes to act on as soon as they happen.
Thanks :-)