There have been a few bug recently with Portal and IWA. We encountered BUG-000126814 at 10.8.0 which also involved Portal and IWA.
I'm not sure who the right person at Esri to address this question to is, but is Esri moving away from supporting IWA through the web adaptor with Portal?
I ask because there's at least a couple of recent bugs regarding it and it sounds like Esri's plans to fix them are not exactly straightforward.