Did you ever resolve this? I've been banging my head against the keyboard trying to resolve what I believe is the same issue. This looks like the problem: https://github.com/goauthentik/authentik/issues/8956 and I don't see any resolutions.

EDIT: After much frustration I realized I didn't have my ingress-nginx marked as the default ingress class. Curious if there's a way to specify the ingress class name on the outpost, but I didn't see anything in my few minutes of looking. Once I made nginx the default it works fine.

Hmmm, I think I ended up getting this working by passing http://172.18.0.1:9000/outpost.goauthentik.io.
I haven't had to look at this in a few, but I'm using nginx proxy manager, and had to target the internal IP over http to get anything from the embedded outpost.

# Increase buffer size for large headers
# This is needed only if you get 'upstream sent too big header while reading response
# header from upstream' error when trying to access an application protected by goauthentik
proxy_buffers 8 16k;
proxy_buffer_size 32k;

# Make sure not to redirect traffic to a port 4443
port_in_redirect off;

location / {
    # Put your proxy_pass to your application here
    proxy_pass $forward_scheme://$server:$port;
    proxy_set_header Host $host;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header X-Forward-For $proxy_add_x_forwarded_for;
    proxy_set_header Upgrade $http_upgrade;
    proxy_http_version 1.1;
    proxy_set_header Connection "upgrade";
    ##############################
    # authentik-specific config
    ##############################
    auth_request     /outpost.goauthentik.io/auth/nginx;
    error_page       401 = @goauthentik_proxy_signin;
    auth_request_set $auth_cookie $upstream_http_set_cookie;
    add_header       Set-Cookie $auth_cookie;

    # translate headers from the outposts back to the actual upstream
    auth_request_set $authentik_username $upstream_http_x_authentik_username;
    auth_request_set $authentik_groups $upstream_http_x_authentik_groups;
    auth_request_set $authentik_email $upstream_http_x_authentik_email;
    auth_request_set $authentik_name $upstream_http_x_authentik_name;
    auth_request_set $authentik_uid $upstream_http_x_authentik_uid;

    proxy_set_header X-authentik-username $authentik_username;
    proxy_set_header X-authentik-groups $authentik_groups;
    proxy_set_header X-authentik-email $authentik_email;
    proxy_set_header X-authentik-name $authentik_name;
    proxy_set_header X-authentik-uid $authentik_uid;
}

# all requests to /outpost.goauthentik.io must be accessible without authentication
location /outpost.goauthentik.io {
    proxy_pass              http://172.18.0.1:9000/outpost.goauthentik.io;
    # ensure the host of this vserver matches your external URL you've configured
    # in authentik
    proxy_set_header        Host $host;
    proxy_set_header        X-Original-URL $scheme://$http_host$request_uri;
    add_header              Set-Cookie $auth_cookie;
    auth_request_set        $auth_cookie $upstream_http_set_cookie;
    proxy_pass_request_body off;
    proxy_set_header        Content-Length "";
}

# Special location for when the /auth endpoint returns a 401,
# redirect to the /start URL which initiates SSO
location @goauthentik_proxy_signin {
    internal;
    add_header Set-Cookie $auth_cookie;
    return 302 /outpost.goauthentik.io/start?rd=$request_uri;
    # For domain level, use the below error_page to redirect to your authentik server with the full redirect path
    # return 302 https://authentik.company/outpost.goauthentik.io/start?rd=$scheme://$http_host$request_uri;
}```

I hit it over the internal http port I think. I'll have to double check.