The solution was reverting my angular.json file back to the way it was pre-upgrade. When I ran ng update @angular/core@18 @angular/cli@18 the updater switch the builder from @angular-devkit/build-angular:browser to @angular-devkit/build-angular:application. This ran fine locally but the Azure Web App was not set up in a way to run with this configuration. As to why the Web App was still displaying the old angular front-end after a successful deploy I am not sure.
