1.1.1.1 is assigned to APNIC Labs in Australia. It isn't always reachable for their own reasons.
Using 1.1.1.1 makes a http request, which lets the captive portal take over, so good job, but you are actually hitting a group that actually exists, and what if they suddenly launch a HTST site on that IP? It might break your method.
Of course this thread makes me wonder if I've been doing it wrong.