IIS server connection to itself for monitoring issue

Help!
1

We have a HAproxy as a replacement for the old NLB, but we are facing an issue.
We have scripts running on the nodes themselves that check if the NLB is working properly.
Now we are seeing some weird behaviour. When we connect from Node1 to our HAproxy to a frontend with ACLs to connect to the correct backend. In this case an API call that exists on both Node1 and Node2. The API call requires Windows Authentication, when we connect the first time it will usually connect to the other node (No issues there). But when we connect with our application it will switch to the other Node (Itself) and then the Authentication popup keeps reappearing and results in authentication failed.

Here are the relevant parts of our config file.

global
        log /dev/log    local0
        log /dev/log    local1 notice
        chroot /var/lib/haproxy
        stats socket /run/haproxy/admin.sock mode 660 level admin
        stats timeout 30s
        user haproxy
        group haproxy
        daemon

        # Default SSL material locations
        ca-base /etc/ssl/certs
        crt-base /etc/ssl/private
        # See: https://ssl-config.mozilla.org/#server=haproxy&server-version=2.0.3&config=intermediate
        ssl-default-bind-ciphers ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA38>        
        ssl-default-bind-ciphersuites TLS_AES_128_GCM_SHA256:TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256
        ssl-default-bind-options ssl-min-ver TLSv1.2 no-tls-tickets

defaults
        log     global
        mode    http
        option  httplog
        option  dontlognull
        option  http-keep-alive
        option  prefer-last-server
        timeout connect 5000
        timeout client  50000
        timeout server  50000
        errorfile 400 /etc/haproxy/errors/400.http
        errorfile 403 /etc/haproxy/errors/403.http
        errorfile 408 /etc/haproxy/errors/408.http
        errorfile 500 /etc/haproxy/errors/500.http
        errorfile 502 /etc/haproxy/errors/502.http
        errorfile 503 /etc/haproxy/errors/503.http
        errorfile 504 /etc/haproxy/errors/504.http

frontend HAproxy80
        bind x.x.x.x:80
        mode http

        ...
        acl url_Api path_beg /Api
        use_backend Api80 if url_Api
        ...

        default_backend app80

backend Api80
        option httpchk
        balance roundrobin
        cookie SERVERID insert indirect nocache
        http-check connect
        http-check send meth GET  uri /Api
        http-check expect ! status 503
        server node01 x.x.x.y:80 check cookie node01
        server node02 x.x.x.z:80 check cookie node02

backend app80
        balance source
        http-check send uri /Test
        http-check expect ! rstatus ^5
        server node01 x.x.x.y:80 check inter 10s rise 2 fall 2
        server node02 x.x.x.z:80 check inter 10s rise 2 fall 2