App runner 404 when using explicit 443 port

I am running a custom container in App Runner. The app is running fine with the default url provided by App Runner. The problem is that when I use the explicit port 443 I get an 404 error.

https://xxx.us-east-1.awsapprunner.com -> This works fine

https://xxx.us-east-1.awsapprunner.com:443 -> 404 Not Found

What is going on here?