I think that it is not a bug, but a feature unexpected thing thar Glitch does. (for instance, it could response a static "No Files" page which says, that a project doesn’t have any .html files/responses for ‘/’)
This is something called an indexing page, it lists out the www directory. This happens when using hello-webpage. I don’t know what the container is running, but it sorta reminds me of running an Apache server. If you don’t have a default entry (index page) it’ll show you the directory listing page.