http://0.0.0.0/<script>
OK, I see that.
My guess is that the Error Console is flooded with too many CSS warnings to be able to keep the XSS messages in the list.
That must be it.
I'd clear the Console, open the link in a new tab, & there are a ton of messages. Just didn't expect it to be of such a number that I would loose the wanted XSS message.