HTTPS vs HTTP, 443 vs 80

there are 2 things to consider here: port number and protocol (http VS https [encrypted http]). These are absolutely independent. It is considered standard to serve http on 80 and https on 443. However, you can configure your webserver to serve any of the protocols on any of the ports, including to serve clear-text requests (http) on port 443. But in most cases, unless you must make life harder for clients, you stick to defaults (clear-text on 80, encrypted on 443).

In key web.page.get[] the first parameter defines protocol to use. If it's not specified, Zabbix will use http. In your second usage you don't specify protocol in the first parameter, so it's http but you override default port (80) to 443, hense Zabbix connects clear-text to 443 and this is unexpected by the webserver.