Accessing Web GUI on Reverse Proxy Not Working

I just upgraded a 91G from 7.4.11 to 7.6.7.

i have been using Nginx Reverse Proxy to access my fortigate web interface and never had an issue. now after upgrading the web interface does not want to load when i go to fortigate.mydomain.com but works perfectly fine when using the fortigate IP address directly.

any insights into what changed?