We're looking to start serving different assets to HTTP/1.x and HTTP/2 (single CSS files and multiple-files respectively). Cloudfront (for example) will serve HTTP/2 to the client, and downgrade the server request to HTTP/1.1 (adding a Via header declaring the client connection as 2.0). The server sees SERVER_PROTOCOL of HTTP/1.1, ignoring the Via header. It should really parse the Via header if it's trusted and use that, or, probably more correctly, have a new Request method to find this value.

On a related note, it looks like the HttpFoundation Response will only reply in HTTP 1.x unless the protocol version is manually set (after prepare() is called).