Why is macOS sending plain HTTP requests to my secure web proxy?

I have developed a web proxy server. It's running on a Raspberry Pi in my house, and it seems to be working as intended using Linux devices I have around and using cURL on my Mac. However, in practice, when I configure my Mac in System Settings to use the proxy server, things don't go as expected. I found that my Mac will always send plain HTTP requests to the server despite having configured a "secure web proxy (HTTPS)" as opposed to a "web proxy (HTTP)" (and the Proxy-Authorization header is missing despite having configured a username and password). I tried handling this in the proxy server, by detecting if it received HTTP data instead of TLS data and responding with a “426 Upgrade Required” and the relevant headers, but this seems to be ignored. The TLS certificate was generated using [mkcert](https://github.com/FiloSottile/mkcert) , and the CA is correctly installed on my Mac. The proxy server is served on port 443. My Mac is on macOS 15.5 (24F74). I also tested on my iPhone, iOS 18.5 (22F76), which seems to completely ignore the proxy altogether. As far as I can tell, no requests from my iPhone go through the proxy at all.