Jonathan Quinn
I was prepared to edit the WebContextURL, but did not do so by default. I attributed this to the hardiness of the esri stack and/or fluke.
All URL's were FQDN during the install & configuration. The only change in this space is that the Portal Web Adaptor URL was configured to the DNS after installation & configuration. In portaladmin/system/webadaptors/<ID>/edit
we have https://DNS_Alias/arcgis, instead of https://FQDN/arcgis.
Standard user behaviour when navigating the 2D map viewer contains DNS alias references to my Portal & DNS alias references to my basemap server. Not a FQDN or broken GET request in sight.