For reasons to many to bring up here, we are trying access a Portal for ArcGIS web adapter using IIS Application Request Routing setup on two load balanced web servers. The web adapter is installed on an internal web server, and the ARR rules are configured on public facing external servers. The issue we are having is that when the request reaches the Portal, we receive a 302 URL Redirect that drops the ARR route value, which causes a 404 not found error.
This is the notional URL pattern using the ARR rule: https://oursite.com/redirect/PortalWebAdapter
This is the response from the 302 redirect https://oursite.com/PortalWebAdapter/home
We know we can avoid using ARR by installing the web adapter on the public facing systems, but only one web adapter can be associated with Portal and we need to have two for the load balanced web servers.