In my browser devtools i can see that the favicon should be available here: <link rel="icon" href="/favicon-f518d411df681dc1e8258eb14b218a97.svg?vsn=d">.
Perhaps the Plug.Static explicit :only option is the cause of this. From the docs:
:only - filters which requests to serve. This is useful to avoid file system traversals on every request when this plug is mounted at "/". For example, if only: ["images", "favicon.ico"] is specified, only files in the “images” directory and the “favicon.ico” file will be served
Try to move the file inside a folder specified in the :only option and test like that.
Or, keep the favicon at the root, but move it to :only_matching:
:only_matching - a relaxed version of :only that will serve any request as long as one of the given values matches the given path. For example, only_matching: ["images", "favicon"] will match any request that starts at “images” or “favicon”, be it “/images/foo.png”, “/images-high/foo.png”, “/favicon.ico” or “/favicon-high.ico”. Such matches are useful when serving digested files at the root. Defaults to nil (no filtering).