Not redirected to gitlab SSO on IIS reverse proxy

hi.

using iis as a reverse proxy

using gitlab SSO (not omnibus)
mattermost domain : mm2.XX.co.kr
gitlab domain : gittest.XX.co.kr

Reverse rewrite host in response headers “check config”

image

not redirected to gitlab login page

packet capture on iis server

No connection attempt from GitLab server packet capture

What is the problem?