Which is not surprising at all, since the whole point of HTTPS is to prevent anyone other than Google from displaying a valid web page at https://www.google.com

Which is not surprising at all, since the whole point of HTTPS is to prevent anyone other than Google from displaying a valid web page at https://www.google.com/ https://www.google.com/ If a captive portal is able to display a valid web page when you try to visit an HTTPS website, that would be a serious red flag.