Deploy web app to Azure but still showing Microsoft page instead

The last time i had an issue like this is was because of the Default Documents. Make sure you default page is listed in here and is above hostingstart.html

In my case the app was failing to start up due to an unhandled exception thrown in my startup code.

However, the reason it was difficult to diagnose is it wasn't shown in the logs in the "log stream" section of the Azure portal. (this is using linux with ASP.NET core 3.1)

When I used the SSH console, went into the /home/LogFiles directory I could see log messages saying what had gone wrong.