In upstream mode, https requests will cause changes in flow.mode

Problem Description

In upstream mode, https requests will cause changes in flow.mode.

Steps to reproduce the behavior:

proxy.py

from mitmproxy import http

def request(flow: http.HTTPFlow) -> None:
    if flow.request.method == "CONNECT":
        return
    if flow.live:
        print("flow.mode:", flow.mode)
        host = "127.0.0.1"
        port = 7890
        flow.live.change_upstream_proxy_server((host, port))

run bash

mitmdump -p 8081
mitmdump --mode upstream:http://127.0.0.1:8081 -s proxy.py --ssl-insecure
export http_proxy=http://127.0.0.1:8080
export https_proxy=http://127.0.0.1:8080

result

$ curl https://httpbin.org/get
...
flow.mode: upstream
...
$ curl https://httpbin.org/get -k
...
flow.mode: transparent
...

System Information

$ mitmproxy --version
Mitmproxy: 4.0.4

I don't know if this is a bug, but I think #3721 and #2123 are related to this issue.

English is not my native language; please excuse typing errors.

Start by reproducing the behavior with the provided mitmdump, proxy.py, and curl commands, comparing requests with and without -k. Trace where flow.mode changes during HTTPS handling and upstream proxy switching; done when HTTPS requests preserve the expected upstream mode without regressing the shown cases.