The above fix didn’t work for me either but the below did:
WP Engine Sage 9 Fix
After a couple of different support agents today one was able to help. Ask your support agent to change the nginx configuration. Here is what he changed it to:
I placed this in the “Before-in-location” portion of the server block
“set $is_trusted 1;”
1 minute later the white screen of death had disappeared 
WPE recently changed their environment structure, and along with it… they changed some of their policies. You can no longer git clone from a WPE server, and they won’t make changes to your Nginx config anymore either.
Since it was time to re-do my own WPE deployment pipeline, I thought it would be nice to create a local environment that matched my remote Gitlab runner, so I could interactively test deploy scripts. Having access to a local config eliminates the need to push a million tweaks to Gitlab.
Here’s a repo that contains instructions on creating your own local environment for building and pushing Sage sites to WPE:
Here’s what you need to tell WPE support to do before pushing to a “production” site install the first time:
We have a tool on our backend that allows us to make caching exemptions… I just added the following from the document you provided:
/wp-content/uploads/cache
then I purged our cache to make sure that was fully in place
and here’s the Gitlab .yml and deploy scripts as I’m using them.
Feel free to ask questions and I’ll try to help.