I've attached a screenshot of the NAT rules. I re-enabled them to block access to the admin webgui, but now my web server is publicly accessible again which was what I was hoping to disable for a while. Maybe I should just delete the NAT rules instead of disabling them?
Sorry, somehow I completely forgot that I have another ISP firewall on the public side of my OPNsense box which has DMZ/Application forwarding set for specific ports (including 443/80) to go to my OPNsense box. I guess I still would have expected the GUI not to show up since the ISP firewall is sending traffic to what is the public/WAN interface on the OPNsense box.
To avoid future confusion: so this is "working as intended"?