https://www.itdroplets.com/iis-php-and-windows-authentication-run-as-a-service-account/

In Section (1), go to system.webServer/serverRuntime and change authenticatedUserOverride from UseAuthenticatedUser to UseWorkerProcessUser (2). Make sure you click on Apply.

Dropping this here so when I forget in the future I can look it up again. This is what resolved the issue for me. :)