I don't know if I'd call it a bug. \u is behaving as advertised. It's upcasing the next char (which happens to be a space) :-)

But, I would agree to a change in spec and implementation that says it upcases the next non-whitespace char when /x is in effect. Too bad you'll have to wait for 5.12 if it's not changed in 5.10 already though as I think it's in feature freeze.