<f:ajax> systematically refreshes the whole page

I am trying this tutorial with the latest version of Payara Community 6.9.0.

Contrary to what the tutorial claims, the <f:ajax> tag always refreshes the whole page, whatever I do. That is, no partial response is ever sent back by the server, only the full page.

How is it possible that even a recent basic tutorial stops working after only two years?

Is this a known issue?